compositor: Use native GL mipmapping instead of MetaTextureTower (v2)
An attempt to follow-through on !1877 (closed), taking review comments into account.
Original commits are untouched, I only tried to move the functionality into its own separate class.
Test setup:
Arch Linux VM using virtio-gpu/virgl
1. gnome-shell --wayland
2. Start firefox with MOZ_ENABLE_WAYLAND=1
3. Un-maximize firefox, move window around (had a debug print showing new code being hit) and went into/out of the overview