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/cube.hlsli

@@ -0,0 +1,13 @@
+
+struct VSInput
+{
+    float3 Position : TEXCOORD0;
+    float3 Color : TEXCOORD1;
+};
+
+struct VSOutput
+{
+    float4 Color : TEXCOORD0;
+    float4 Position : SV_Position;
+};
+