reset y panning when moving left or right

This makes this pan mode really useful, because you don't need
to go all the way to top of page when you get to bottom of first
column.

pdfreader.lua

@@ -453,11 +453,17 @@ function PDFReader:inputloop()
 					if self.offset_x > 0 then
 						self.offset_x = 0
 					end
+					if self.pan_by_page then
+						self.offset_y = 0
+					end
 				elseif ev.code == KEY_FW_RIGHT then
 					self.offset_x = self.offset_x - x
 					if self.offset_x < self.min_offset_x then
 						self.offset_x = self.min_offset_x
 					end
+					if self.pan_by_page then
+						self.offset_y = 0
+					end
 				elseif ev.code == KEY_FW_UP then
 					self.offset_y = self.offset_y + y
 					if self.offset_y > 0 then