[Feature request] [Overlays] Touch anywhere for analog input

A great improvement to retroarch touch overlays would be the ability to touch anywhere to activate analog input. Many modern touch-based games implement this behavior, for example the Square Enix FF and DQ titles on mobile. To describe the feature specifically, the player may touch anywhere on the screen to start an analog input, with the center of the analog being the initial touch point, and the player can move around this center to send an analog input in that direction.

Retroarch can implement this behavior in the negative space left in an overlay. Touching into negative space would temporarily block input to other buttons on the overlay, allowing the player to touch over other buttons and not activate them, as long as they continue to hold the touch.

This feature should be implemented in retroarch by a setting such as: Allow touching anywhere for analog input: Off | Left | Right | Both. Both would split the screen vertically between left and right analog inputs.
Players wishing to use the touch anywhere feature for a d-pad should be able to do so by turning on the analog-to-digital setting.

The issue names no files or tests. Start by tracing RetroArch's touch-overlay input handling and the existing analog-to-digital setting, then determine where the new Off/Left/Right/Both option belongs. Done means touch-anywhere analog input works in the selected region without activating overlay buttons, and the behavior is covered by suitable tests or manual verification.