When line smoothing is enabled, the driver now increases the width of
the line so that it can add some semi-transparent pixels to either side
of the line. A lowering pass is added which modifies the alpha component
of every write to fragment output 0 so that if the fragment is outside
the width of the line then the alpha is reduced. It additionally
discards fragments that are completely invisible. It might seem bad to
use discard on a tiled renderer but the assumption is that any bad
effects from using discard will also happen anyway because of enabling
alpha blending.

v2: Disable the line smoothing pass entirely when the framebuffer
    contains an integer colour output or one with no alpha channel.
    Calculate the coverage once upfront and store in a global variable
    instead of calculating each time an output write is modified. Also
    do the conditional discard once upfront.
v3: Don’t check whether the output buffer has an alpha channel. Only
    look at output 0. Use aa_line_width intrinsic instead of calculating
    the real line width in the shader. Clamp the coverage as part of the
    global variable, not per output write.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5624>