skvx: allow more implicit conversions

Guarding the implict constructors and scalar/vector
operations with std::is_convertible ought to make SkVx
types feel more like normal C types, allowing implicit
conversions exactly when the scalar equivalents would.

This shouldn't change the behavior of any code, or make
anything new possible... just nicer to read and write.

Change-Id: Iff4b89012c5b8c7f7933e6841c925b81186bc614
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201402
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
2 files changed