### v3d backend

This implements some of v3d using the closed source v3dv3 tree's
C/C++-based simulator.  All execution is synchronous.

Export: `MESA_LOADER_DRIVER_OVERRIDE=v3d
LD_PRELOAD=$prefix/lib/libv3d_drm_shim.so`.  The v3dv3 version exposed
will depend on the v3dv3 build -- 3.3, 4.1, and 4.2 are supported.

### v3d_noop backend

This implements the minimum of v3d in order to make shader-db work.
The submit ioctl is stubbed out to not execute anything.

Export `MESA_LOADER_DRIVER_OVERRIDE=v3d
LD_PRELOAD=$prefix/lib/libv3d_noop_drm_shim.so`.  This will be a V3D
4.2 device.

### vc4_noop backend

This implements the minimum of vc4 in order to make shader-db work.
The submit ioctl is stubbed out to not execute anything.

Export `MESA_LOADER_DRIVER_OVERRIDE=vc4
LD_PRELOAD=$prefix/lib/libvc4_noop_drm_shim.so`.  This will be a VC4
2.1 device.

