Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_screen_space_antialiasing.txt

+Name
+
+    CHROMIUM_screen_space_antialiasing
+
+Name Strings
+
+    GL_CHROMIUM_screen_space_antialiasing
+
+Version
+
+    Last Modifed Date: August 27, 2015
+
+Dependencies
+
+    OpenGL ES 2.0 is required.
+
+Overview
+
+    This extension performs the antialiasing to all color attachments of
+    the currently bound draw framebuffer. Reference GL_INTEL_framebuffer_CMAA
+    for more details.
+
+New Tokens
+
+    None
+
+New Procedures and Functions
+
+    void ApplyScreenSpaceAntialiasingCHROMIUM(void);
+
+    Apply antialiasing to all color attachments of the currently bound draw framebuffer.
+
+    INVALID_OPERATION is generated if any of the current draw buffers is multisampled.
+
+    OUT_OF_MEMORY may be generated if fail to allocate internal resources necessary to 
+    perform the requested operation.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+Revision History
+
+    27/8/2015    Documented the extension