gpu: drop Framebuffer.IsComplete in favour of an error from NewFramebuffer

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

gpu/backend.go

@@ -20,7 +20,7 @@ type Backend interface {
 	IsTimeContinuous() bool
 	NewTexture(format TextureFormat, width, height int, minFilter, magFilter TextureFilter, bindings BufferBinding) Texture
 	DefaultFramebuffer() Framebuffer
-	NewFramebuffer(tex Texture) Framebuffer
+	NewFramebuffer(tex Texture) (Framebuffer, error)
 	NewImmutableBuffer(typ BufferBinding, data []byte) Buffer
 	NewBuffer(typ BufferBinding, size int) Buffer
 	NewProgram(vertexShader, fragmentShader ShaderSources) (Program, error)
@@ -127,7 +127,6 @@ type Framebuffer interface {
 	Bind()
 	Invalidate()
 	Release()
-	IsComplete() error
 }
 
 type Timer interface {