es2tri asserts when GL_OES_mapbuffer is not available

The code shows:

218     #if 1 /* test code */
219        proc p = eglGetProcAddress("glMapBufferOES");
220        assert(p);
221     #endif

This is wrong without first checking that the GL_OES_mapbuffer extension is exposed, and it does not seem to be useful for this demo.