Remove 'in' variables from SkRuntimeEffect

Runtime effects previously allowed two kinds of global input variables:
'in' variables could be bool, int, or float. 'uniform' could be float,
vector, or matrix. Uniform variables worked like you'd expect, but 'in'
variables were baked into the program statically. There was a large
amount of machinery to make this work, and it meant that 'in' variables
needed to have values before we could make decisions about program
caching, and before we could catch some errors. It was also essentially
syntactic sugar over the client just inserting the value into their SkSL
as a string. Finally: No one was using the feature.

To simplify the mental model, and make the API much more predictable,
this CL removes 'in' variables entirely. We no longer need to
"specialize" runtime effect programs, which means we can catch more
errors up front (those not detected until optimization). All of the API
that referred to "inputs" (the previous term that unified 'in' and
'uniform') now just refers to "uniforms".

Bug: skia:10593
Change-Id: I971f620d868b259e652b3114f0b497c2620f4b0c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309050
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
18 files changed