Added way to select and edit colors of placed Tris.
Added third_party catch unit test framework that builds the UnitTest in
Debug builds.
Refactor internal use of notification_text.
Rename src/imgui_helper.hpp to src/helpers.hpp
Added some helper functions, including "is_within_shape" (used for
selecting a Tri).
Fixed use of flags in helpers not using enum values.
Increased buffer for notificationText. Previous implementation wrote to
notificationText but was possible for buffer overflows when writing to
it with strcpy. Also fixed by using strncpy instead of strcpy.
This is to prevent slowdown when drawing per frame. Previously, the
program would draw each individual triangle each frame. Now the draws
are cached to a texture that is drawn each frame, and the cache is
recreated on click or undo/redo events.
