HU_Drawer Improvements

[Corbachu]

Up to 3 (4?) lines of HU_Drawer messages, each one fades in instantly (like a pickup flash), then fades out slower. Ref hu_stuff.cc.

that should be doable. I'd just store an array of the 4 most recent messages, as well as a gametimestamp of when they were created. for every frame the array is read, figuring out which is the oldest message, and then discarding any that are X too old, and for any that are X-70 tics newer, start the fade process for said messages progressing from (X-70 to X) interpolating 1.0 to 0.0 on the way down.
it is also not necessary to interpolate this because even 35tics/sec fading looks very smooth