New button texture, small code cleanup

Added new .dds textures for UI purposes.
Removed redundant (and exceedingly large) hexagons texture.
The CombatMap class is no longer initialized with the screen dimensions
as the class makes no use of that data.
Initial CombatMap UI

Implemented basic (visual) UI to the CombatMap in the form of a menu
across the bottom of the screen.
Adjusted camera bounds to allow for the entire map to be seen with the
addition of the UI.
Added SetDimensions function to the Bitmap class to allow for resizing
of a bitmap (and its texture) without reinitialization or the use of a
second instance.
Repositioned the return to main menu functionality on the CombatMap to
the bottom right button on the UI, providing a visual marker for the
functionality.