panfrost: Bring up shader-db

Alyssa Rosenzweig requested to merge alyssa/mesa:shaderdb into master

First, we add a debug option for precompiling shaders so shader-db isn't totally inert. We then fix a bug regarding empty shaders identified in shader-db. We finally add support for 2D shadow samplers, which are required by some shaders in the library in GLSL 1.20 mode. With this, we are able to do runs of shader-db proper. Currently a run (in GLSL 1.20 mode, mind you) takes a bit more than 30s on RK3399. The next step will be cutting this.

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