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.frag.hlsl

@@ -0,0 +1,10 @@
+
+#include "overlay.hlsli"
+
+Texture2D<float4> t0 : register(t0, space2);
+SamplerState s0 : register(s0, space2);
+
+float4 main(VSOutput input) : SV_Target0
+{
+    return t0.Sample(s0, input.uv);
+}