Correct clamping of TEXTURE_{MAX, BASE}_LEVEL

Correct clamping of TEXTURE_{MAX, BASE}_LEVEL

Previously, if TEXTURE_IMMUTABLE_FORMAT was TRUE, the levels were allowed to
be set like usual, but ARB_texture_storage states:

> if TEXTURE_IMMUTABLE_FORMAT is TRUE, then level_base is clamped to the range
> [0, <levels> - 1] and level_max is then clamped to the range [level_base,
> <levels> - 1], where <levels> is the parameter passed the call to
> TexStorage* for the texture object

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Corey Richardson <corey@octayn.net>