misc fixes for VC4

[Eric Anholt]

I've been looking into X performance on VC4 recently.  The first
obvious thing happening was that we're hitting some fallbacks in the
driver for things like GL_QUADS, so I thought "what if I use the GLES2
paths instead?"  Turns out the GLES2 paths are thoroughly broken now.
I wrote two quick patches that clean up a bunch of errors, but it's
not working yet.

The big performance win, though, is fixing copyarea to give the GL
information about the area that might be damaged by the operation,
using the scissor.  Given that it's negative lines of code and not
significant on i965, I think this is a pretty good idea.  I do wonder
if we don't want to just always leave scissoring on, like I did to
logic op.

I'm not sure if we want to slip these in to the current release,
though -- #1 is touchy enough that it might regress the normal case,
and it's not like GLES2 actually ends up working.  I'd be fine with
delaying until the merge window reopens.