Multiply the cursor position by the current scaling mode multiplier when
getting the color under the cursor. Also multiply the screen rectangle
before checking if the cursor is within bounds.
Color picker would not account for scaling when getting the color under
the cursor.
Fixes#13906.
With this change you can now set the theme and background color at the
same time in the Display Settings. Before if both were changed
before hitting 'apply' the theme background color would overwrite
the custom background.
