Add SK_ALWAYS_INLINE.

Add SK_ALWAYS_INLINE.

I'm working on some code that's much faster when compiled by GCC than by Clang
because GCC inlines more aggressively.  Using SK_ATTRIBUTE(always_inline) on
the appropriate methods narrows the performance gap considerably.

This should work for MSVC, GCC, and Clang, otherwise falling back to "inline".

BUG=
R=reed@google.com

Committed: https://code.google.com/p/skia/source/detail?r=12364

[reed1, 2013-11-22 16:47:16]

can you add a simple e.g. in the comments, to show how to use this new define?

lgtm

[mtklein, 2013-11-22 16:56:12]

On 2013/11/22 16:47:16, reed1 wrote:
> can you add a simple e.g. in the comments, to show how to use this new define?
> 
> lgtm

Done.

[mtklein, 2013-11-22 16:56:25]

Committed patchset #3 manually as r12364 (presubmit successful).