Add method for frontends to query the backend's cursor location.

The Rockbox frontend allows games to be displayed in a "zoomed-in"
state targets with small displays. Currently we use a modal interface
-- a "viewing" mode in which the cursor keys are used to pan around
the rendered bitmap; and an "interaction" mode that actually sends
keys to the game.

This commit adds a midend_get_cursor_location() function to allow the
frontend to retrieve the backend's cursor location or other "region of
interest" -- such as the player location in Cube or Inertia.

With this information, the Rockbox frontend can now intelligently
follow the cursor around in the zoomed-in state, eliminating the need
for a modal interface.

bridges.c

@@ -2146,6 +2146,20 @@ struct game_drawstate {
     bool started, dragging;
 };
 
+
+static void game_get_cursor_location(const game_ui *ui,
+                                     const game_drawstate *ds,
+                                     const game_state *state,
+                                     const game_params *params,
+                                     int *x, int *y, int *w, int *h)
+{
+    if(ui->cur_visible) {
+        *x = COORD(ui->cur_x);
+        *y = COORD(ui->cur_y);
+        *w = *h = TILE_SIZE;
+    }
+}
+
 /*
  * The contents of ds->grid are complicated, because of the circular
  * islands which overlap their own grid square into neighbouring
@@ -3267,6 +3281,7 @@ const struct game thegame = {
     game_redraw,
     game_anim_length,
     game_flash_length,
+    game_get_cursor_location,
     game_status,
     true, false, game_print_size, game_print,
     false,			       /* wants_statusbar */