Added gl::windowToWorldCoord() and gl::worldToWindowCoord() utility functions.
Added a test application for gl::windowToObjectCoord() et. al.
Made using gl::windowToObjectCoord() easier...
...by allowing the user to specify the model matrix. That way, you don't have to set it as the active model matrix. It also allows us to inline gl::windowToWorldCoord(). Updated the test application so it exercises all functions.
Changed ivec2 to vec2. Removed icons.
Merge pull request #544 from paulhoux/window-to-world

Added gl::windowToWorldCoord() and gl::worldToWindowCoord() functions.