SDL: add multitouch support

[Frenzie]

Partially inspired by the Android implementation.

After I added it, I suddenly realized we could've had fake multitouch on desktop for years. There's a right button on the mouse, after all. A possible alternative would be to simply fake right mouse button as being two fingers at once, as opposed to having to truly press left and right at the same time.

---

This change is 

[Frenzie]

@ptrm Btw, could you check if removing these lines works as expected in combination with the check for SDL_TOUCH_MOUSEID? It might require a "new" version of SDL in case you still happen to be running one from, say, five years ago.

koreader-base/ffi/SDL2_0.lua

Lines 295 to 300 in 263b211

	-- if we got an event, examine it here and generate events for koreader
	if ffi.os == "OSX" and (event.type == SDL.SDL_FINGERMOTION or
	event.type == SDL.SDL_FINGERDOWN or
	event.type == SDL.SDL_FINGERUP) then
	-- noop for trackpad finger inputs which interfere with emulated mouse inputs
	do end -- luacheck: ignore 541

[NiLuJe]

Untested, but nothing jumps out at a quick glance ;).

[Frenzie]

Rest assured I'd have noticed if anything didn't work. ;-)

@ptrm Have you been able to try the Apple touchpad? I noticed you left a thumbs up :-P

[Frenzie]

I tested macOS 13.3.1 myself. It seems to be fine.

[NiLuJe]

This may be called for !is_finger, while it will access event.tfinger.fingerId, I'm not sure what it points to in the SDL magic union in those cases ;).

[Frenzie]

Oh right, a minor oversight for the mouse, though it doesn't really matter in practice.

[Frenzie]

The most obvious would probably be:

diff --git a/ffi/SDL2_0.lua b/ffi/SDL2_0.lua
index 4823dcac..ee5a767d 100644
--- a/ffi/SDL2_0.lua
+++ b/ffi/SDL2_0.lua
@@ -235,8 +235,9 @@ local function getFingerSlot(event)
 end
 
 local function genTouchDownEvent(event, slot, x, y)
+    local is_finger = event.type == SDL.SDL_FINGERDOWN
     genEmuEvent(C.EV_ABS, C.ABS_MT_SLOT, slot)
-    genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, tonumber(event.tfinger.fingerId))
+    genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, is_finger and tonumber(event.tfinger.fingerId) or slot)
     genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X, x)
     genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y, y)
     genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)

I prefer tracking id not to be constantly 0 and 1 'cause if nothing else it's clearer to read. 😅

[ptrm]

Rest assured I'd have noticed if anything didn't work. ;-)

@ptrm Have you been able to try the Apple touchpad? I noticed you left a thumbs up :-P

Sorry, that was meant as a quick sign of queueing it ;) will try it today, I don't have a touchpad on my mac mini though, so it'll be just mouse.

[Frenzie]

Additional tests are welcome, but I did test the touchpad on a Macbook Air M1 already. It didn't seem to generate finger events.

Looking again that used to be the behavior at some point in the past (and clearly it was back ca. 2014 when that code was written), but it looks like more recent SDL has made it configurable:

libsdl-org/SDL@bdc7f95

Going back a little further, it was four years ago that the touchpad behavior was changed for Mac, see libsdl-org/SDL@e841b06 and a little further back libsdl-org/SDL@d9a2eff

So I think we might want to set SDL_HINT_TRACKPAD_IS_TOUCH_ONLY for best results in SDL 2.23+ (?) but the rest of the code should work fine either way regardless on any SDL from at least the past four years. And if there are any lingering issues with synthesized touch or mouse events they'll probably occur across platforms.