Moved 3d_render_src contents into src

To simplify building and debugging the 3D rendering code, I moved it
into the main src/ directory. This breaks the AIR 2.6 build, but an
upcoming change will fix that.
Reformatted code
Wrapped 3d code with conditional compilation flag

The 'SCRATCH::allow3d' flag now controls 3D-related code.
Fixed Ant build WRT SCRATCH::allow3d compile flag
Fixed improper 'this' in bitmap2d() local function

Thanks @sclements for finding this problem
Updated Travis build file to build with/without 3D
Added missing function(){} wrapper
Merge pull request #507 from LLK/refactor3d

Refactor3d