BACKENDS: Refactor OpenGL & SDL graphics backends

This patch refactors the OpenGL and SDL graphics backends,
primarily to unify window scaling and mouse handling, and to
fix coordinate mapping between the ScummVM window and the
virtual game screen when they have different aspect ratios.

Unified code for these two backends has been moved to a new
header-only WindowedGraphicsManager class, so named because it
contains code for managing graphics managers that interact with
a windowing system and render virtual screens within a larger
physical content window.

The biggest behavioral change here is with the coordinate
system mapping:

Previously, mouse offsets were converted by mapping the whole
space within the window as input to the virtual game screen
without maintaining aspect ratio. This was done to prevent
'stickiness' when the mouse cursor was within the window but
outside of the virtual game screen, but it caused noticeable
distortion of mouse movement speed on the axis with blank
space.

Instead of introducing mouse speed distortion to prevent
stickiness, this patch changes coordinate transformation to
show the system cursor when the mouse moves outside of the virtual
game screen when mouse grab is off, or by holding the mouse inside
the virtual game screen (instead of the entire window) when mouse
grab is on.

This patch also improves some other properties of the
GraphicsManager/PaletteManager interfaces:

* Nullipotent operations (getWidth, getHeight, etc.) of the
  PaletteManager/GraphicsManager interfaces are now const
* Methods marked `virtual` but not inherited by any subclass have
  been de-virtualized
* Extra unnecessary calculations of hardware height in
  SurfaceSdlGraphicsManager have been removed
* Methods have been renamed where appropriate for clarity
  (setWindowSize -> handleResize, etc.)
* C++11 support improved with `override` specifier added on
  overridden virtual methods in subclasses (primarily to avoid
  myself accidentally creating new methods in the subclasses
  by changing types/names during refactoring)

Additional refactoring can and should be done at some point to
continue to deduplicate code between the OpenGL and SDL backends.
Since the primary goal here was to improve the coordinate mapping,
full refactoring of these backends was not completed here.

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