fix warnings on Mac in src/gpu

fix warnings on Mac in src/gpu and include/gpu

Fix these class of warnings:
- unused functions
- unused locals
- sign mismatch
- missing function prototypes
- missing newline at end of file
- 64 to 32 bit truncation

The changes prefer to link in dead code in the debug build
with 'if (false)' than to comment it out, but trivial cases
are commented out or sometimes deleted if it appears to be
a copy/paste error.
Committed: https://code.google.com/p/skia/source/detail?r=4180

[bsalomon, 2012-06-05 19:10:53]

A bunch of the dead funcs and vars can just be deleted. However, I'd say check
in as is and those I care about I can remove later.

LGTM