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.vert.hlsl

@@ -0,0 +1,25 @@
+
+#include "overlay.hlsli"
+
+static const uint verts[6] = {0, 1, 2, 0, 2, 3};
+static const float2 uvs[4] = {
+    {0.0f, 0.0f},
+    {1.0f, 0.0f},
+    {1.0f, 1.0f},
+    {0.0f, 1.0f}
+};
+static const float2 pos[4] = {
+    {-1.0f, 1.0f},
+    {1.0f, 1.0f},
+    {1.0f, -1.0f},
+    {-1.0f, -1.0f}
+};
+
+VSOutput main(uint id : SV_VertexID)
+{
+    VSOutput output;
+    uint vert = verts[id];
+    output.uv = uvs[vert];
+    output.pos = float4(pos[vert], 0.0f, 1.0f);
+    return output;
+}