enable 3rd texture unit on radeon (default still 2 enabled units). Disable some multi...

enable 3rd texture unit on radeon (default still 2 enabled units). Disable some multitexcoord codegen stuff noone understands to make it work. Replace most code testing explicitly for unit 0 and 1 with loops instead of adding test for unit 2, smaller/more readable code at the cost of maybe some slight performance hit. (Code provided by Andreas Stenglein, some adjustments by me.)