Runtime SkSL: Reflect inputs in SkRuntimeEffect

Adds a new structure to describe the inputs, including type,
shape, name, etc. This removes any references to the type
variables in the compiler's context, and centralizes the
logic for parsing those type variables.

Also normalizes the rules for what types are supported.
(This was inconsistent among the various functions before).

Now:
  - bool and int must be 'in'
  - float may be 'in' or 'uniform'
  - float[2-4] and matrices must be 'uniform'

Includes a new Slide that allows for interactive editing
of a runtime shader.

Change-Id: Ic2af68a80c8bb645ba96417c430da24de0a9c2d8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260497
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
9 files changed