Made some fixes to touchscreen control detection:
* TouchScreenMode is activated on the first SDL2 FingerDown
* TouchScreenMode deactivates after the last finger is removed, and a
mouse event happens at least 5 ticks later.
* Touchscreen mode used to be detected based on SDL2 GetNumTouchDevices
but on a Macbook, the trackpad registers as a touch device - worse,
GetNumTouchDevices will only start returning 1 the first time some
devices are touched.
* The result was that on macOS the custom mouse cursor was drawn by
default, but on the first trackpad touch, would disappear in favor of
assuming the game is running on a touch screen device (which is not
the case).
* New method: the render engine has an IsFingerDown boolean which will
be true as long as at least one finger has registered a FingerDown
event, but not yet a FingerUp event.
* So as long as one finger is down, the mouse cursor can disappear and
then it comes back on release. This isn't perfectly ideal for pure
touch devices (ideally the cursor remains hidden until a mouse
movement without touch occurs).
