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

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.