intel: Clarify first_level/last_level vs baselevel/maxlevel by deletion.

intel: Clarify first_level/last_level vs baselevel/maxlevel by deletion.

This has always been ugly about our texture code -- object base/max
level vs intel object first/last level vs image level vs miptree
first/last level.  We now get rid of intelObj->first_level which is
just tObj->BaseLevel, and make intelObj->_MaxLevel clearly based off
of tObj->_MaxLevel instead of duplicating its code (incorrectly, as
image->MaxLog2 only considers width/height and not depth!)