meta: Don't try to glOrtho when the draw buffer isn't initialized.

meta: Don't try to glOrtho when the draw buffer isn't initialized.

I ran across this while running a glGenerateMipmap() test.

_meta_GenerateMipmap sets MESA_META_TRANSFORM, which causes
_mesa_meta_begin to try and set a default orthographic projection.

Unfortunately, if the drawbuffer isn't set up, ctx->DrawBuffer->Width
and Height are 0, which just causes an GL_INVALID_VALUE error.

Fixes oglconform's fbo/mipmap.automatic, mipmap.manual, and
mipmap.manualIterateTexTargets.

Reviewed-by: Brian Paul <brianp@vmware.com>