There are two places we need to do color conversion.
1. When upload a image data to a texture.
2. When download a texture to a memory buffer.
As the color format may not be supported in GLES2. We may
need to do the following two operations to convert dat.
a. revert argb to bgra / abgr to rgba.
b. swap argb to abgr / bgra to rgba.
Signed-off-by: Zhigang Gong <zhigang.gong@gmail.com>