meta: Track the _mesa_meta_DrawPixels VBO just like the others

meta: Track the _mesa_meta_DrawPixels VBO just like the others

All of the other meta routines have a particular pattern for creating
and tracking the VAO and VBO.  This one function deviated from that
pattern for no apparent reason.

Almost all of the code added in this patch will be removed shortly.

v2: Drop glDeleteBuffers() of the old, now-uninitialized vbo variable.
    Fixes getteximage-formats and fbo-mipmap-copypix regression when "2"
    landed in the variable (change by anholt).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>