SkSplicer support for 2-point gradients.

Add some stages to SkSplicer:
   - seed_shader
   - matrix_2x3
   - clamp_x
   - clamp_y
   - linear_gradient_2stops

seed_shader needed new constants, 0.5f and {0,1,2,3,4,5,6,7}.

$ out/nanobench -m gradient_linear_clamp\$ --config f16 --ms 2000 -q
    Before: 612.17us
    After:  163.80us

Change-Id: I6e03383c95ea070250424e743080a7930efeca77
Reviewed-on: https://skia-review.googlesource.com/7348
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed