Implementation of a GL-virtualization layer for Skia. This allows for

Implementation of a GL-virtualization layer for Skia.  This allows for
environments using skia to specify a GL implementation at run-time, instead of
relying on the linker to pull in the appropriate GL impl.

A new structure, GrGLInterface is exposed.  This struct contains a set of
function pointers that should point to an appropriate GL implementation.

This change also removes the reliance on GLew on windows builds.

Review: http://codereview.appspot.com/4254059/

git-svn-id: http://skia.googlecode.com/svn/trunk@937 2bbb7eff-a529-9590-31e7-b0007b416f81