draw: simplify (and correct) aaline fallback (v2)

The motivation actually was to get rid of the additional tex
instruction, since that requires the draw fallback code to intercept
all sampler / view calls (even if the fallback is never hit).
Basically, the idea is to use coverage of the pixel to calculate
the alpha value, and coverage is simply based on the distance
to the center of the line (in both line direction, which is useful
for wide lines, as well as perpendicular to the line).
This is much closer to what hw supporting this natively actually does.
It also fixes an issue with line width not quite being correct, as
well as endpoints getting stretched too far (in line direction) with
wide lines, which is apparent with mesa demo line-sample.
(For llvmpipe, it would probably make sense to do something like this
directly when drawing lines, since rendering two tris is twice as
expensive as a line, but it would need some changes with state
management.)
Since we're no longer relying on mipmapping to get the alpha value,
we also don't need to draw 3 rects (6 tris), one is sufficient.

There's still issues (as before):
- quite sure it's not correct without half_pixel_center, but can't test
this with GL.
- aaline + line stipple is incorrect (evident with line-sample demo).
Looking at the spec the stipple pattern should actually be based on
distance (not just dx or dy for x/y major lines as without aa).
- outputs (other than pos + the one used for line aa) should be
reinterpolated since we actually increase line length by half a pixel
(but there's no tests which would care).

v2: simplify the math (should be equivalent), don't need immediate
v3: use float versions of atan2,cos,sin, minor cleanups

Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>

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