A bunch of little SkMScalar-as-float build fixes.

Landing this before http://crrev.com/1169813006 so the test changes needed
there are clearer.  This just lets things build with floats, and is a no-op for
doubles.

No public API changes we didn't intend to already have...
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1173673003
4 files changed