gpu/internal/d3d11: stub BlitFramebuffer for D3D11

The compute renderer doesn't run on Windows yet, but the d3d11 backend needs
the method to satisfy the driver interface.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/driver/driver.go

@@ -46,6 +46,7 @@ type Device interface {
 	BindIndexBuffer(b Buffer)
 	BindImageTexture(unit int, texture Texture, access AccessBits, format TextureFormat)
 
+	BlitFramebuffer(dst, src Framebuffer, srect, drect image.Rectangle)
 	MemoryBarrier()
 	DispatchCompute(x, y, z int)