_m_a_s_k Specifies a bit mask to enable and disable writing of individual
bits in the stencil planes. Initially, the mask is all 1's.
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
ggggllllSSSStttteeeennnncccciiiillllMMMMaaaasssskkkk controls the writing of individual bits in the stencil
planes. The least significant n bits of _m_a_s_k, where n is the number of
bits in the stencil buffer, specify a mask. Where a 1 appears in the
mask, it's possible to write to the corresponding bit in the stencil
buffer. Where a 0 appears, the corresponding bit is write-protected.
Initially, all bits are enabled for writing.
EEEERRRRRRRROOOORRRRSSSS
GGGGLLLL____IIIINNNNVVVVAAAALLLLIIIIDDDD____OOOOPPPPEEEERRRRAAAATTTTIIIIOOOONNNN is generated if ggggllllSSSStttteeeennnncccciiiillllMMMMaaaasssskkkk is executed between
the execution of ggggllllBBBBeeeeggggiiiinnnn and the corresponding execution of ggggllllEEEEnnnndddd.