glsl: make use of on disk shader cache

glsl: make use of on disk shader cache

The hash key for glsl metadata is a hash of the hashes of each GLSL
source string.

This commit uses the put_key/get_key support in the cache put the SHA-1
hash of the source string for each successfully compiled shader into the
cache. This allows for early, optimistic returns from glCompileShader
(if the identical source string had been successfully compiled in the past),
in the hope that the final, linked shader will be found in the cache.

This is based on the intial patch by Carl.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>