fix two finger pan gestures detection

This is a stupid bug I introduced when I thought 0 evaluated to be false.
Actually in Lua only explicit `false` and `nil` are evaluated to `false`
all other values are true in logical expressions.

This should also fix #546. Probably because the touch screens in Kindle
are not as sensitive as in Kobo, this problem is not discovered early
util I play with multi-touch on a Android phone which has a sensitive
enough screen to expose this bug.

frontend/ui/gesturedetector.lua

@@ -460,7 +460,7 @@ function GestureDetector:handleTwoFingerPan(tev)
     -- triggering slot
     local tslot = tev.slot
     -- reference slot
-    local rslot = tslot and 0 or 1
+    local rslot = tslot == 1 and 0 or 1
     local tpan_dir, tpan_dis = self:getPath(tslot)
     local tstart_pos = Geom:new{
         x = self.first_tevs[tslot].x,

koreader-base

@@ -1 +1 @@
-Subproject commit 0047fa7aa34f686641abfa3db7e11217d2315137
+Subproject commit 2b38b16d2795a228b1a541fbc2e09bd093119aad