Filter empty frames on the receiver

In !108 (merged) the code is added to filter on the sender sided, but the receiver side is missing. Empty frames should be filtered before being added to the event queue, i.e. this sequence:

TOUCH_UP
FRAME
FRAME
FRAME
TOUCH_DOWN
FRAME

Should result in

EIS_EVENT_TOUCH_UP
EIS_EVENT_FRAME
EIS_EVENT_TOUCH_DOWN
EIS_EVENT_FRAME

(and of course the same for the ei side of things)