vbo: unmap vertex store before executing lists

vbo: unmap vertex store before executing lists

We don't want our VBOs mapped when we're drawing.  This change checks
if the vertex store VBO is mapped before we execute a list, unmaps it,
then remaps it after drawing.  This situation pops up when building a
nested display list in GL_COMPILE_AND_EXECUTE mode.

Reviewed-by: Eric Anholt <eric@anholt.net>