SDL event queue overhelming

[cepetr]

If touch_read() function isn't called frequently enough (<10ms), the SDL event queue can be became overhelmed by SDL_MOUSEMOTION events. This can result in delayed UI reactions and unexpected behaviour.

We didn't observe this behaviour until I began experimenting with animations on the display (when call to touch_read() are less frequent).

A simple solution is to merge succesive SDL_MOUSEMOTION events into one when the touch_read() function is called.