v3d: Add a "precompile" debug flag for shader-db.

v3d: Add a "precompile" debug flag for shader-db.

I've been using my apitrace-based shader-db so far, but it's slow
(apitrace decompression), intrusive (apitrace windows spamming the
screen), and doesn't have much coverage.  The original shader-db provides
a lot more coverage and compiles faster, at the expense of not having the
actual runtime variant key.  As v3d has a lot less runtime variation than
vc4 did, this tradeoff makes more sense.