add a new public header for Mesa

Merged Chia-I Wu requested to merge olv/virglrenderer:venus-hw into master

Per #175 (closed), virgl_hw.h is considered legacy and new things not virgl-specific should go into a new header.

This MR adds virglrenderer_hw.h. The header consists of all definitions of opaque data consumed by virglrenderer.h and is a companion header of virglrenderer.h. It is a superset of virgl_hw.h (and future venus_hw.h).

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