use bitmasks instead of pure enums where appropriate

[rursprung]

i just ran into this with: MouseEventFlags: in most cases the flags parameter will be set to 0 in the callback triggered by highgui::set_mouse_callback. but 0 is not a valid value according to the enum, thus try_from will return an Err.

using something like bitmask-enum (i've never personally used this crate directly) might resolve this, as it'd also correctly allow setting multiple flags at the same time.

[twistedfall]

Thank you for the suggestion, I'll investigate!