gpu: replace Backend.DefaultFramebuffer by Backend.CurrentFramebuffer

DefaultFramebuffer was set up at Backend creation time, which is
difficult to predict. Instead, let GPU query and cache the current
FBO when created.

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

gpu/backend.go

@@ -19,7 +19,7 @@ type Backend interface {
 	// are valid at the point of call.
 	IsTimeContinuous() bool
 	NewTexture(format TextureFormat, width, height int, minFilter, magFilter TextureFilter, bindings BufferBinding) (Texture, error)
-	DefaultFramebuffer() Framebuffer
+	CurrentFramebuffer() Framebuffer
 	NewFramebuffer(tex Texture, depthBits int) (Framebuffer, error)
 	NewImmutableBuffer(typ BufferBinding, data []byte) (Buffer, error)
 	NewBuffer(typ BufferBinding, size int) (Buffer, error)