winex11.drv: fshack: Interpolate looked up colour in gamma shader.

CW-Bug-Id: #20400

dlls/winex11.drv/opengl.c

@@ -2149,12 +2149,19 @@ static const char *fs_hack_gamma_frag_shader_src =
 "\n"
 "layout(location = 0) out vec4 outColor;\n"
 "\n"
+"vec3 color_from_index(vec3 index)\n"
+"{\n"
+"    ivec3 i = ivec3(index);\n"
+"    return vec3(values[i.r].r, values[i.g].g, values[i.b].b);\n"
+"}\n"
+"\n"
 "void main(void)\n"
 "{\n"
-"    vec4 lookup = texture(tex, texCoord) * 255.0;\n"
-"    outColor.r = values[int(lookup.r)].r;\n"
-"    outColor.g = values[int(lookup.g)].g;\n"
-"    outColor.b = values[int(lookup.b)].b;\n"
+"    vec3 lookup = texture(tex, texCoord).xyz * 255.0;\n"
+"    vec3 lookup1, lookup2;\n"
+"    lookup1 = floor(lookup);\n"
+"    lookup2 = ceil(lookup);\n"
+"    outColor.xyz = mix(color_from_index(lookup1), color_from_index(lookup2), lookup - lookup1);\n"
 "    outColor.a = 1.0;\n"
 "}\n"
 ;