Added an initial skeleton program.

.github/workflows/main.yml

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+name: Build
+
+on: [push, pull_request]
+
+jobs:
+  Build:
+    name: ${{ matrix.platform.name }}
+    runs-on: ${{ matrix.platform.os }}
+
+    strategy:
+      matrix:
+        platform:
+        - { name: Linux,   os: ubuntu-latest }
+        - { name: Windows, os: windows-latest }
+        - { name: MacOS,   os: macos-latest }
+
+    steps:
+    - name: Set up SDL
+      id: sdl
+      uses: libsdl-org/setup-sdl@v1
+      with:
+        version: sdl3-head
+    - name: Get project sources
+      uses: actions/checkout@v3
+    - name: Configure CMake
+      run: cmake -B build ${{ matrix.platform.flags }}
+    - name: Build
+      run: cmake --build build/

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12.0)
+project(SDL_helloworld)
+
+find_package(SDL3 CONFIG REQUIRED)
+
+add_executable(sdl-helloworld
+    main.c
+)
+
+target_link_libraries(sdl-helloworld PRIVATE SDL3::SDL3-shared)
+target_link_libraries(sdl-helloworld PUBLIC SDL3::Headers)
+
+if (WINDOWS)
+    set_property(TARGET sdl-helloworld PROPERTY WIN32_EXECUTABLE TRUE)
+endif()
+

LICENSE.txt

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+All code in this sample is public domain. Do whatever you want with it.
+
+We recommend you replace this text with your own license, unless you also
+want your code to be public domain.
+
+
+

README.md

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+# SDL_helloworld
+
+This is a simple skeleton program that builds an extremely simple SDL-based
+program, setting up CI for GitHub Actions and such.
+
+Feel free to use this as a starting point for your own code.
+

main.c

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+#include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
+
+static SDL_Window *window = NULL;
+static SDL_Renderer *renderer = NULL;
+
+static int setup_program(int argc, char **argv)
+{
+    if (SDL_Init(SDL_INIT_VIDEO) == -1) {
+        SDL_Log("SDL_Init(SDL_INIT_VIDEO) failed: %s", SDL_GetError());
+        return -1;
+    }
+
+    window = SDL_CreateWindow("Hello SDL", 640, 480, 0);
+    if (!window) {
+        SDL_Log("SDL_CreateWindow() failed: %s", SDL_GetError());
+        return -1;
+    }
+
+    renderer = SDL_CreateRenderer(window, NULL, 0);
+    if (!renderer) {
+        SDL_Log("SDL_CreateRenderer() failed: %s", SDL_GetError());
+        return -1;
+    }
+
+    return 0;
+}
+
+static void mainloop(void)
+{
+    SDL_FRect mouseposrect;
+    Uint8 r;
+    SDL_bool keep_going = SDL_TRUE;
+    SDL_Event event;
+
+    mouseposrect.x = mouseposrect.y = -1000;  /* -1000 so it's offscreen at start */
+    mouseposrect.w = mouseposrect.h = 50;
+
+    /* run the program until told to stop. */
+    while (keep_going) {
+
+        /* run through all pending events until we run out. */
+        while (SDL_PollEvent(&event)) {
+            switch (event.type) {
+                case SDL_EVENT_QUIT:  /* triggers on last window close and other things. End the program. */
+                    keep_going = SDL_FALSE;
+                    break;
+
+                case SDL_EVENT_KEY_DOWN:  /* quit if user hits ESC key */
+                    if (event.key.keysym.sym == SDLK_ESCAPE) {
+                        keep_going = SDL_FALSE;
+                    }
+                    break;
+
+                case SDL_EVENT_MOUSE_MOTION:  /* keep track of the latest mouse position */
+                    /* center the square where the mouse is */
+                    mouseposrect.x = event.motion.x - (mouseposrect.w / 2);
+                    mouseposrect.y = event.motion.y - (mouseposrect.h / 2);
+                    break;
+            }
+        }
+
+        /* fade between shades of red every 3 seconds, from 0 to 255. */
+        r = (Uint8) ((((float) (SDL_GetTicks() % 3000)) / 3000.0f) * 255.0f);
+        SDL_SetRenderDrawColor(renderer, r, 0, 0, 255);
+
+        /* you have to draw the whole window every frame. Clearing it makes sure the whole thing is sane. */
+        SDL_RenderClear(renderer);  /* clear whole window to that fade color. */
+
+        /* set the color to white */
+        SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
+
+        /* draw a square where the mouse cursor currently is. */
+        SDL_RenderFillRect(renderer, &mouseposrect);
+
+        /* put everything we drew to the screen. */
+        SDL_RenderPresent(renderer);
+    }
+}
+
+static void shutdown_program(void)
+{
+    SDL_DestroyRenderer(renderer);
+    SDL_DestroyWindow(window);
+    SDL_Quit();
+}
+
+
+/* this is always a main() function, even if you're on a platform that uses
+   something else. SDL_main.h takes care of figuring out the details and
+   making sure the program starts here. */
+int main(int argc, char **argv)
+{
+    if (setup_program(argc, argv) == 0) {
+        mainloop();
+    }
+    shutdown_program();
+    return 0;
+}
+