Fix various warnings

Merged Chia-I Wu requested to merge olv/virglrenderer:fix-warnings into master

This fixes all warnings on my system.

I am not sure about the last commit. The warnings caught potential snprintf truncations, but to trigger those warnings means we need to use hard coded values. An alternative is to turn all those into runtime errors

ret = snprintf(...);
assert(ret >= 0 && ret < buf_size);

by adding a wrapper to snprintf. Thoughts?

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