Added support for using the GPU renderer as an offscreen renderer

SDL_CreateGPURenderer() now allows passing in an existing GPU device and passing in a NULL window to create an offscreen renderer.

Also renamed SDL_SetRenderGPUState() to SDL_SetGPURenderState().

test/testgpu/overlay.hlsli

@@ -0,0 +1,5 @@
+struct VSOutput
+{
+    float2 uv : TEXCOORD0;
+    float4 pos : SV_Position;
+};