Backends: SDLGPU3: Added sdl_gpu backend. (#8163, #7998, #7988)

+Squashed: Optimized shader source code encoding by ocornut (#8163, #7998, #7988)
(squashed to avoid storing both in git history, 130 KB->62 KB)

backends/sdlgpu3/shader.frag

@@ -0,0 +1,14 @@
+#version 450 core
+layout(location = 0) out vec4 fColor;
+
+layout(set=2, binding=0) uniform sampler2D sTexture;
+
+layout(location = 0) in struct {
+    vec4 Color;
+    vec2 UV;
+} In;
+
+void main()
+{
+    fColor = In.Color * texture(sTexture, In.UV.st);
+}