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.