Provide WEBGL_draw_buffers functions also with EXT suffix.

The extension is based on EXT_draw_buffers ES2 extension, which has
functions with EXT suffix.

src/library_gl.js

@@ -6437,6 +6437,11 @@ var LibraryGL = {
 
     GL.currentContext.drawBuffersExt(bufArray);
   },
+
+  // OpenGL ES 2.0 draw buffer extensions compatibility
+
+  glDrawBuffersEXT: 'glDrawBuffers',
+
   // signatures of simple pass-through functions, see later
 
   glActiveTexture__sig: 'vi',