Add glVertex3i and glVertex2i calls

These are basically just aliases for the glVertex3f function.
Javascript seems to convert the integers to float values correctly.
However, I did not follow the code path through the immediate mode
emulation to know for sure. In my applications it works for "normal", small
values (e.g. 1, 500, 1000, ...).
Merge pull request #887 from MichaelRiss/glVertex2i

Add glVertex3i and glVertex2i calls