gl/msaa: Properly destroy stencil buffer clip cache

When replacing the stencil buffer clip cache or destroying a surface,
destroy the cached clip. This prevents the clip from leaking.
gl/msaa: Disable stencil and scissor during framebuffer blit

When blitting the framebuffer during transitions to and from
multi-sampling mode, we need to disable the stencil and scissor test so
that the entire surface is preserved. This fixes the bitmap-font test
for the MSAA compositor.
gl/msaa: Always use scissor when clipping

Even when using the stencil buffer for clipping, always scissor the clip
extents. This simplifies the code a bit.
Merge branch 'master' of git://anongit.freedesktop.org/git/cairo