glsl: Improve accuracy of alpha scaling in advanced blend lowering.

glsl: Improve accuracy of alpha scaling in advanced blend lowering.

When blending with GL_COLORBURN_KHR and these colors:

   dst = <0.372549027, 0.372549027, 0.372549027, 0.372549027>
   src = <0.09375, 0.046875, 0.0, 0.375>

the normalized dst value became 0.99999994 (due to precision problems
in the floating point divide of rgb by alpha).  This caused the color
burn equation to fail the dst >= 1.0 comparison.  The blue channel would
then fall through to the dst < 1.0 && src >= 0 comparison, which was
true, since src.b == 0.  This produced a factor of 0.0 instead of 1.0.

This is an inherent numerical instability in the color burn and dodge
equations - depending on the precision of alpha scaling, the value can
be either 0.0 or 1.0.  Technically, GLSL floating point division doesn't
even guarantee that 0.372549027 / 0.372549027 = 1.0.  So arguably, the
CTS should allow either value.  I've filed a bug at Khronos for further
discussion (linked below).

In the meantime, this patch improves the precision of alpha scaling by
replacing the division with (rgb == alpha ? 1.0 : rgb / alpha).  We may
not need this long term, but for now, it fixes the following CTS tests:

ES31-CTS.blend_equation_advanced.blend_specific.GL_COLORBURN_KHR
ES31-CTS.blend_equation_advanced.blend_all.GL_COLORBURN_KHR_all_qualifier

Cc: currojerez@riseup.net
Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://cvs.khronos.org/bugzilla/show_bug.cgi?id=16042
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>