Add an option to support for GL_OES_EGL_image_external.

Comes with this extension is the new sampler type samplerExternalOES.

ANGLEBUG=175
TEST=compile the attached shader file
Review URL: http://codereview.appspot.com/4809076

git-svn-id: https://angleproject.googlecode.com/svn/trunk@728 736b8ea6-26fd-11df-bfd4-992fa37f6226
16 files changed