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.vert.hlsl

@@ -0,0 +1,15 @@
+
+#include "cube.hlsli"
+
+cbuffer UBO : register(b0, space1)
+{
+    float4x4 ModelViewProj;
+};
+
+VSOutput main(VSInput input)
+{
+    VSOutput output;
+    output.Color = float4(input.Color, 1.0f);
+    output.Position = mul(ModelViewProj, float4(input.Position, 1.0f));
+    return output;
+}