screen: Add _GTK_FRAME_EXTENTS to _NET_SUPPORTED
texture-tower: Remove CPU codepath for mipmap generation

The CPU codepath for mipmapping is unusably slow, and we expect modern
graphics cards with modern TFP/FBO support.
tower: make sure not to blend when updating tower

Each level in the tower is initialized by binding the texture for that
level to an offscreen framebuffer and rendering the previous level as a
textured rectangle. The problem was that we were blending the previous
level with undefined data so argb32 windows with transparencies would
result in artefacts. This makes sure to disable blending when drawing
the textured rectangle.