Move SkSLSampleMatrix to include/private

Switch it to use std::string (not SkSL::String). Along these lines,
remove SkSL::String's operator SkString, and instead add an explicit
SkString constructor from std::string.

Other changes aren't strictly necessary, but I wanted to clean up some
of the other SkSL vs. Skia code barriers. VS (since 2015) has had
vsnprintf that conforms to the C99 standard, the only difference with
_vsnprintf is a different (nonstandard) return value for overflowing
calls. Remove the special-case (the only use of SKSL_BUILD_FOR_WIN).

Change-Id: I8826af10c8e78a8d935c601d00b8ae9ba0640041
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298816
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
14 files changed