Merge work from multiple contributors on the mesa package. This
includes the following changes and decisions:
* Added in mesa +hwrender support
* make +swrender the default selection since this represents a very
common usage case (ie, cluster usage)
* leave ~llvm as the default, since inadvertently building this package
could take a very long time. May need to revisit this assumption in
the future since the performance difference can be quite substantial.
* build libGLESv1+2 and libGL for all variants
* avoid dependence on dri/libdrm when ~hwrender
* disable gallium drivers if none are requested
2f269c3919