simpler uniforms() api

By making uniforms() just append to the uniform vector, there's no more
need for two passes, no need for the blitter to ever even know how many
uniforms the effects use, and the effects now never need to deal with a
nullptr uniform buffer.  Much simpler all around.

While we're refactoring, convert the uniform buffer de jure to
std::vector<uint32_t>, which is what we'd been treating the old
std::vector<uint8_t> as by convention, and switch the program() offset
parameter type to size_t as a reminder that it's measured in bytes.

Cq-Include-Trybots: skia.primary:Test-Debian9-Clang-GCE-CPU-AVX2-x86_64-Debug-All-SK_USE_SKVM_BLITTER
Change-Id: I81d2c92aae37a650104f384f815df78c8a186270
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252776
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed