Keith Packard authored May 07, 2014 and Emma Anholt committed Jun 15, 2014

miZeroLine allocates enough space to draw a line spanning the entire
width/height of the target drawable. When drawing multiple shorter
lines, this leaves most of the space in that buffer unfilled. Let
multiple lines be drawn into the buffer if there is plenty of space.

Speeds up glamor fallback zero-width lines:

Before
    6000000 trep @   0.0020 msec (508000.0/sec): 1-pixel line
    6000000 trep @   0.0020 msec (492000.0/sec): 10-pixel line
    6000000 trep @   0.0023 msec (427000.0/sec): 100-pixel line
    4000000 trep @   0.0035 msec (282000.0/sec): 500-pixel line

After:
  600000000 trep @   0.0000 msec (43400000.0/sec): 1-pixel line
  140000000 trep @   0.0001 msec (13000000.0/sec): 10-pixel line
   16000000 trep @   0.0008 msec (1300000.0/sec): 100-pixel line
    4000000 trep @   0.0038 msec (261000.0/sec): 500-pixel line

(500 pixel lines do not change in performance because the buffer can
only one one of them.)

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>