Addition of GL_CHROMIUM_copy_texture extension.

Re-submission of http://codereview.chromium.org/9968113

This version corrects the problem that caussed the revert of the above issue.  The shaders used by the extension were specifying a precision, which is not supported on desktop GL.  The shaders now conditionallyspecify the precision. 

Introduction of CHROMIUM_copy_texture extension that respects pixel-store semantics, and allows copying of BGRA textures.

OpenGL ES does not natively allow for copying textures with a BGRA format. The EXT_texture_format_BGRA8888 extension does not specify support for glCopyTexImage calls on these textures.

This extension provides a routine to perform texture copies to/from BGRA-backed textures that also respects the following CHROMIUM pixel storage modifiers:
UNPACK_FLIP_Y_CHROMIUM
UNPACK_PREMULTIPLY_ALPHA_CHROMIUM

This extension will be useful for the following purposes:
- Copying accelerated Canvas2D contents to WebGL textures without a software readback. (And potentially the same for video-webgl texture copies.)
- Copying Canvas2D contents to the compositor backing store.

BUG=101051
TEST=none

[Jeff Timanus, 2012-04-19 20:41:07]

Hi Gregg & Al,

Could one of you please give this revision a quick look.  It is exactly the same
as that which was reverted, except for the changes to the shaders in
gles2_cmd_copy_texture_chromium.cpp.

Thanks,

Jeff

[apatrick_chromium, 2012-04-19 20:45:30]

lgtm

[commit-bot: I haz the power, 2012-04-20 15:18:33]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/twiz@chromium.org/10124016/12002

[commit-bot: I haz the power, 2012-04-20 17:06:35]

Change committed as 133213