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).
The gamepad mouse cursor has become THE mouse cursor. It is always visible and your
real cursor is hidden, and this way the game can swap out other cursors for certain
scenarios:
* The Pencil Tool in the editor will use a pencil cursor over the level canvas.
* The Flood Tool has a custom Flood cursor so you don't forget it's selected!
Other improvements:
* The Palette buttons in the editor now render using their swatch's pattern
instead of only using its color.
* If you have an ultra HD monitor and open a Bounded level in the editor which
is too small to fill your screen, the editor canvas limits its size to fit
the level (preferable over showing parts of the level you can't actually play
as it's out of bounds).
* The "brush size" box is only drawn around the cursor when a relevant tool is
selected (Pencil, Line, Rect, Ellipse, Eraser)
