SkSL: Allow invoking children (shaders, etc) like functions

Previously, you would declare child objects (shaders, colorFilters, etc.)
and "sample" them like this:

    uniform shader input;
    uniform colorFilter filter;
    half4 main(float2 coord) {
        half4 inColor = sample(input, coord);
        return sample(filter, inColor);
    }

With the new syntax, those child objects become directly callable,
reflecting the way that Skia assembles all parts of the paint (as functions)
in the overall fragment shader:

    uniform shader input;
    uniform colorFilter filter;
    half4 main(float2 coord) {
        half4 inColor = input(coord);
        return filter(inColor);
    }

Bug: skia:12302
Change-Id: Ia12351964dc5d2300660187933188e738671cd83
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436517
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
12 files changed