Add subscribeUniform extension pipeline

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_subscribe_uniform.txt

+Name
+
+    CHROMIUM_subscribe_uniform
+
+Name Strings
+
+    CHROMIUM_subscribe_uniform
+
+Version
+
+    Last Modifed Date: October 15, 2014
+
+Dependencies
+
+    OpenGL ES 2.0 is required.
+
+Overview
+
+    Allows clients to subscribe to a set of input uniforms which will be
+    populated in their shaders when they call
+    glPopulateUniformSubscriptionCHROMIUM and on subsequent calls to 
+    useProgram if the uniform state is dirty.
+
+    Decreases percieved latency for operations performed against these
+    uniforms.
+
+New Tokens
+
+    Accepted by the <target> parameter of SubscribeUniformChromium:
+
+    MOUSE_POSITION_CHROMIUM                                  0x924B
+
+New Procedures and Functions
+
+    The command   
+
+        void glSubscribeUniformCHROMIUM (GLint location, GLenum target)
+
+    Associates an internal uniform with the uniform specified by location
+    in the current bound program.
+    <location> Is the location of the uniform they wish to have populated
+               in the currently bound program.
+    <target> Is the uniform the client wants to have populated.
+
+   The command
+
+       void glUnsubscribeUniformCHROMIUM (GLint location)
+
+   Unsubscribed the uniform specified by location in the current bound 
+   program.
+   <location> Is the location of the uniform they wish to unsubscribe 
+              from in the currently bound program.
+
+   The command
+
+       void glPopulateUniformSubscriptionCHROMIUM ()
+
+   Activates the pending uniform state and updates the current program.
+   Subsequent calls to glUseProgram will use the updated uniform
+   state to populate their subscribed uniforms. 
+   Clients should populate the uniform once per frame to maintain per 
+   frame uniform consistency.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+Revision History
+
+    10/17/2014    Documented the extension