Clamp editor mouse coordinates

The editor has never clamped mouse bounds, and instead relied on SDL to
return clamped mouse coordinates. Unfortunately, in #1140, this detail
was missed, and the behavior changed, allowing the cursor to leave the
bounds of the screen.

This isn't much of a problem most of the time because anything that
uses position as array indeces has bounds checks. Unfortunately,
placing entities outside of a room's bounds leads to entities you can't
ever touch ever again without modifying the level externally. This is
the worst with checkpoints, as pressing Enter in a room will now take
you to the out-of-bounds checkpoint, causing you to immediately wrap to
another room.

This fix should be backported to 2.4, as the issue can lead to level
files which need manual editing to fix!

desktop_version/src/Editor.cpp

@@ -3175,8 +3175,8 @@ void editorinput(void)
     ed.old_tilex = ed.tilex;
     ed.old_tiley = ed.tiley;
 
-    ed.tilex = key.mousex / 8;
+    ed.tilex = SDL_clamp(key.mousex, 0, SCREEN_WIDTH_PIXELS - 1) / 8;
-    ed.tiley = key.mousey / 8;
+    ed.tiley = SDL_clamp(key.mousey, 0, SCREEN_HEIGHT_PIXELS - 1) / 8;
 
     bool up_pressed = key.isDown(SDLK_UP) || key.isDown(SDL_CONTROLLER_BUTTON_DPAD_UP);
     bool down_pressed = key.isDown(SDLK_DOWN) || key.isDown(SDL_CONTROLLER_BUTTON_DPAD_DOWN);