We should be uploading any vertex data using this kind of upload
style, since it saves a bunch of extra copies of our vertex data.

v2:
  - Add a simple comment about what the function does.
  - Use get_vbo_space()'s return in trapezoids, instead of dereffing
    glamor_priv->vb (by Markus Wick).
  - Fix the double-unmapping by moving put_vbo_space() outside of
    flush_composite_rects().
  - Remove the rest of the composite_vbo_offset usage, and just always
    use get_vbo_space()'s return value.
v3:
  - Fix failure to put_vbo_space in traps when no prims were
    generated.
  - Unbind the VBO from put_vbo_space().  Keeps callers from
    forgetting to do so.
v4:
  - Split out some changes into the previous 3 commits while trying to
    track down a regression.
  - Fix regression due to rebase fail where glamor_priv->vbo_offset
    wasn't incremented.
v5:
  - Fix GLES2 VBO sizing.
  - Add a comment about resize behavior.
  - Move glamor_vbo.c init code to glamor_vbo.c from
    glamor_render.c. (Derived from Markus's changes, but the GLES2 fix
    dropped almost all of the code in the functions).
v6:
  - Drop the initial BufferData on GLES2 (it happens at put() time).
  - Don't forget to set vbo_offset to the size on GLES2.
  - Use char * instead of void * in the cast to return the vbo_offset.
  - Resize the default FBO to 512kb, to be similar to previous
    behavior.  +1.66124% +/- 0.284223% (n=679) on aa10text.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Markus Wick <markus at selfnet.de>