svga: implement an index buffer translation cache

svga: implement an index buffer translation cache

Some OpenGL apps, like Cinebench R15, have many glDrawElements(GL_QUADS)
calls.  Since we don't directly support quads we have to convert these
calls into GL_TRIANGLES which involves generating a new index buffer.

This patch saves the new/translated index buffer in the hope that it
can be reused for a later draw call.

Cinebench R15 increases by about 20% with this change.
The NobelClinician Viewer app also hits this code.
Tested with full piglit run.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>