Properly implement dragging a rotated rotated map
Don't hardware the window size to 1024 by 768

Rather query the window state
Switch from a hardcoded constant to a define

Improves the understandibility. Also switch to 65° because it looks better.
Fix scaling of dragging when rotated
Make the window resizeable

Resizing breaks pretty much everything hardcoded and will be fixed at a later point
Clean up global namespace by creating some structs
Move input handling from main.cpp to its own file