This adds support for TearFree page flips to eliminate tearing without the
use of a compositor. It allocates two shadow buffers for each CRTC, a back
buffer and a front buffer, and uses damage tracking to minimize excessive
copying between buffers and skip unnecessary flips when the screen's
contents remain unchanged. It works on transformed screens too, such as
rotated and scaled CRTCs.

When PageFlip is enabled, TearFree won't force fullscreen DRI clients to
synchronize their page flips to the vblank interval.

TearFree is disabled by default.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>