We were calling this from the CI span function, but not the RGBA
span function.
I don't know of a test program for the GL_EXT_depth_bounds_test
extension...
The integer Z clamping range depends on the number of bits
in the Z buffer because that's the scale factor used when we
transform NDC coords by the viewport/depth range.
Fixes fd.o bug #25972 but only for Z buffers up to a depth
of 30 bits. Beyond that we get into messy integer overflow
issues and things fall apart.
