shaped-texture: Don't clear mipmaps during painting

This reverts an attempt at saving texture memory that was introduced recently in 16fa2100. It was misguided because the same texture may be needed in the next frame if a window has multiple previews visible on screen at once (gnome-shell's overview). Keeping the mipmaps around seems to reduce the peak render times of the overview by roughly 5%-10%.

CC @rmader
Supersedes !2595 (closed)