Mouse position and button events in timeline

[binaryf]

Would be nice to have mouse position and button events forwarded to the timeline like keyboard events. this means we can forward mouse events to effects for various purposes.

[einarf]

Mouse positions are easy to forward. They are already present, but not forwarded. The missing part is finding a way to unify the mouse button events because they are handled in fairly different ways between the Qt, GLFW etc. Should not be too hard.