First commit for the new Mac OS X project

Converted the C rendering functions to Objective-C / Cocoa.  Added in
the definitions for methods present in the super classes such as
drawRect:, mouseDown:, etc. The code to handle rotations, translations
and other matrix stuff isn’t in yet - that’s the stuff I’m working on
now. There’s a basic drawing function that clears the scene and puts a
triangle up there. It’s pretty rough right now, but things are drawing
on the screen.
Update .gitignore to ignore DerivedData

The DerivedData directory is where build products are placed, along
with temporary and intermediate data. We don’t want this stuff tracked
in git.