Bitmap: Don't unbind shader after render op

Since all rendering is now shader based, there's normally
no reason to ever unbind a shader program.

src/bitmap.cpp

@@ -748,8 +748,6 @@ void Bitmap::hueChange(int hue)
 
 	p->blitQuad(quad);
 
-	shader.unbind();
-
 	p->popViewport();
 
 	TEX::unbind();