glsl2: Fix the implementation of atan(y, x).

glsl2: Fix the implementation of atan(y, x).

So many problems here.  One is that we can't do the quadrant handling
for all the channels at the same time, so we call the float(y, x)
version multiple times.  I'd also left out the x == 0 handling.  Also,
the quadrant handling was broken for y == 0, so there was a funny
discontinuity on the +x side if you plugged in obvious values to test.

I generated the atan(float y, float x) code from a short segment of
GLSL and pasted it in by hand.  It would be nice to automate that
somehow.

Fixes:
glsl-fs-atan-1
glsl-fs-atan-2