use same signature for shader/colorfilters in sksl

'void main(float x, float y, inout half4 color)'

For colorfilters, x,y are uninitialized
For shaders, color is paint-color

Change-Id: I21c2f570702f117d33646bd8fba0ab4def20a118
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227439
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
9 files changed