SDL3/SDL_SetRenderViewport: Updated.

[SDLWikiBot]

This edit was made by @sechshelme.

Live page is here: https://wiki.libsdl.org/SDL3/SDL_SetRenderViewport

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[slouken]

[1/1] Building C object CMakeFiles/SDL3_SDL_SetRenderViewport_c.dir/wikidir/examples/SDL3/SDL_SetRenderViewport.c.o
FAILED: CMakeFiles/SDL3_SDL_SetRenderViewport_c.dir/wikidir/examples/SDL3/SDL_SetRenderViewport.c.o 
/usr/bin/cc -D__ANDROID__ -I/home/runner/work/sdlwiki/sdlwiki/.github/../external/SDL3/include -I/home/runner/work/sdlwiki/sdlwiki/.github/../external/SDL3/src/video/khronos -isystem /usr/lib/jvm/temurin-11-jdk-amd64/include -isystem /usr/lib/jvm/temurin-11-jdk-amd64/include/linux  -MD -MT CMakeFiles/SDL3_SDL_SetRenderViewport_c.dir/wikidir/examples/SDL3/SDL_SetRenderViewport.c.o -MF CMakeFiles/SDL3_SDL_SetRenderViewport_c.dir/wikidir/examples/SDL3/SDL_SetRenderViewport.c.o.d -o CMakeFiles/SDL3_SDL_SetRenderViewport_c.dir/wikidir/examples/SDL3/SDL_SetRenderViewport.c.o -c /home/runner/work/sdlwiki/sdlwiki/build/wikidir/examples/SDL3/SDL_SetRenderViewport.c
/home/runner/work/sdlwiki/sdlwiki/build/wikidir/examples/SDL3/SDL_SetRenderViewport.c: In function ‘main’:
/home/runner/work/sdlwiki/sdlwiki/build/wikidir/examples/SDL3/SDL_SetRenderViewport.c:17:32: error: ‘nullptr’ undeclared (first use in this function)
   17 |   SDL_RenderFillRect(renderer, nullptr);
      |                                ^~~~~~~
/home/runner/work/sdlwiki/sdlwiki/build/wikidir/examples/SDL3/SDL_SetRenderViewport.c:17:32: note: each undeclared identifier is reported only once for each function it appears in
ninja: build stopped: subcommand failed.