This mostly just takes every draw call and turns it into a sequence of
commands that clear the FBO and draw a single shaded triangle to it,
regardless of the actual input vertices or shaders.  I copied the initial
driver skeleton mostly from freedreno, and I've preserved Rob Clark's
copyright for those.  I also based my initial hardcoded shaders and
command lists on Scott Mansell (phire)'s "hackdriver" project, though the
bit patterns of the shaders emitted end up being different.

v2: Rebase on gallium megadrivers changes.
v3: Rebase on PIPE_SHADER_CAP_MAX_CONSTS change.
v4: Rely on simpenrose actually being installed when building for
    simulation.
v5: Add more header duplicate-include guards.
v6: Apply Emil's review (protection against vc4 sim and ilo at the same
    time, and dropping the dricommon drm bits) and fix a copyright header
    (thanks, Roland)