Dispmanx is the prorietary display API on the Raspberry Pi, which
provides hardware compositing. Every visible surface is assigned a
Dispmanx element, and the hardware or firmware will do all compositing
onto screen. The API supports translation, scaling, flips, discrete
rotations in 90-degree steps, alpha channel on the surfaces, and
full-surface alpha on top.

Previously, Dispmanx capabilities were used via the weston_plane
mechanism, where surfaces were assigned to planes when possible, and
otherwise transparently falling back to GLESv2 compositing. Because we
have no way to use the same memory buffer as a GL texture and a Dispmanx
resource, we had to prepare for both. In the worst case, that means one GL
texture, and two (double-buffered case) Dispmanx resources, all the size
of a whole surface, for all surfaces. This was eating memory fast. To
make things worse (and less slow), the wl_shm buffer was kept around,
since it was copied to either a texture or a resource as nee...