texenv_combine: Add OPERAND2_ALPHA support for ONE_MINUS_SRC_ALPHA

This is feature is not part of EXT_texture_env_combine, but it was added by basically every extension that came after. All of the ATI_texture_env_combine3 test cases in this test try to use this feature, and they all fail with:

Problem in combine(): bad alphaOperand2

This fix is necessary so that ext_texture_env_combine-combine can get to the failure mentioned in mesa#6872.