Fix Indirect command struct definitions

[LtSten]

The documentation differs from the GL 4.5 (or equivalently 4.6) spec in its definition of both DrawArraysIndirectCommand and DrawElementsIndirectCommand. In particular:

• DrawArraysIndirectCommand has no primCount member, but rather an instanceCount member, which in turn matches that of glDrawArraysInstancedBaseInstance; c.f. GL4.5 §10.4, p353 or GL4.6 §10.4, p366.
• As above for DrawElementsIndirectCommand. Moreover, baseVertex is an int, not a uint; c.f. GL4.5 §10.4, p359 or GL4.6 §10.4, p373.

[oddhack]

@pdaniell-nv please signoff - LGTM. This is a change we made a while back in the spec / XML but didn't propagate here.