iris: Change vendor and renderer strings

iris: Change vendor and renderer strings

This patch changes the GL_VENDOR string from "Mesa Project" to "Intel".
This makes GLX_MESA_query_renderer report "Vendor: Intel (0x8086)"
instead of "Vendor: Mesa Project (0x8086)" which is arguably wrong.
We now also use a consistent vendor string across Windows and Linux.

It also prepends "Mesa" to the GL_RENDERER string, both to credit the
community and have a distinguishing mark between the two drivers.  We
drop "DRI" compared to i965, as it's not really that important.

Improves performance in Portal by 1.8x.  Iris is now 3.86% faster
than i965 at the portal-d1.dem timedemo on my Kabylake laptop.  One
change is that Portal selects the MapBufferRange path based on the
vendor string, and iris's BufferSubData path is still missing the
storage invalidation optimization.