set front and back function and reference value for stencil testing
$funcSpecifies the test function. Eight symbolic constants are valid:
GL_NEVER, GL_LESS, GL_LEQUAL, GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL, and GL_ALWAYS. The initial value is GL_ALWAYS.
$refSpecifies the reference value for the stencil test. ref is clamped to the range 0 2 n - 1 , where n is the number of bitplanes in the stencil buffer. The initial value is 0.
$maskSpecifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done. The initial value is all 1's.
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