st/nine: Cache constant buffer size

st/nine: Cache constant buffer size

The shader constant buffer size with the
constant compaction code can vary depending
on the shader variant compiled (for example if
fog constants are required, etc).
Thus instead of using fixed size for the shader,
add in the variant cache the size required, pass it
to the context, and use this value.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>