Ignore sub-pixel mouse motion

To avoid slowdowns due to unnecessary repeated events at the same
pixel location, when using high sample rate, high DPI gaming mouse.

Patch by Pere.  h/t Sadie for reporting the issue & sending a video.

docs/CHANGES.txt

@@ -6,7 +6,7 @@ Copyright (c) 2002-2025
 Various contributors (see below, and AUTHORS.txt)
 https://tuxpaint.org/
 
-2025.September.28 (0.9.36)
+2025.November.9 (0.9.36)
   * Documentation updates:
     ----------------------
     + Describe where Tux Paint searches for system fonts,
@@ -22,6 +22,17 @@ https://tuxpaint.org/
       Bill Kendrick <bill@newbreedsoftware.com>
       h/t Will Thompson for pointing this out
 
+  * Other Improvements:
+    -------------------
+    + Expose SDL2_gfx to Magic Tool plugins
+      Will Thompson <https://sourceforge.net/u/wjjjjt/profile/>
+
+    + Tux Paint discards sub-pixel mouse motion (which can cause
+      slow-down when using an input device with very high DPI and
+      polling rate, e.g. a gaming mouse).
+      Pere Pujal i Carabantes <perepujal@gmail.com>
+      h/t Sadie for reporting
+
   * Bug Fixes:
     ----------
     + When Magic tools are not grouped ("ungroupmagictools"),
@@ -40,12 +51,6 @@ https://tuxpaint.org/
       Bill Kendrick <bill@newbreedsoftware.com>
       h/t Wisdom Lupus
 
-  * Other Improvements:
-    -------------------
-    + macOS build updated to remove dependency on libSDLmain.a no longer
-      required, a holderover from SDL 1.2.
-      Mark Kim <markuskimius@gmail.com>
-
   * Localization Updates:
     ---------------------
     + French translation
@@ -62,8 +67,9 @@ https://tuxpaint.org/
 
   * Ports & Building:
     -----------------
-    + Expose SDL2_gfx to Magic Tool plugins
+    + macOS build updated to remove dependency on libSDLmain.a no longer
-      Will Thompson <https://sourceforge.net/u/wjjjjt/profile/>
+      required, a holderover from SDL 1.2.
+      Mark Kim <markuskimius@gmail.com>
 
 2025.May.26 (0.9.35)
   * New Magic Tools:

src/tuxpaint.c

@@ -22,7 +22,7 @@
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   (See COPYING.txt)
 
-  June 14, 2002 - June 6, 2025
+  June 14, 2002 - November 9, 2025
 */
 
 #include "platform.h"
@@ -2691,6 +2691,12 @@ static void mainloop(void)
 
     while (SDL_PollEvent(&event))
     {
+      if (event.type == SDL_MOUSEMOTION && oldpos_x == (int)event.motion.x && oldpos_y == (int)event.motion.y)
+        {
+          DEBUG_PRINTF("Discarding mouse motion event\n");
+          break;
+        }
+
       current_event_time = SDL_GetTicks();
 
       /* To avoid getting stuck in a 'catching up with mouse motion' interface lock-up */