[RFC] Responsive fill mode

Problem

PlayCanvas currently supports:
• FILLMODE_KEEP_ASPECT and FILLMODE_FILL_WINDOW: tied to the window size
• FILLMODE_NONE: allows manual control over the canvas dimensions

However, a common use case - embedding a responsive canvas that fills its container (width: 100%; height: 100%) - requires manually tracking container size and managing updates with FILLMODE_NONE. This is unintuitive and boilerplate-heavy for something that should be simple.

related - https://github.com/playcanvas/react/issues/123

Proposal

Introduce a new fill mode:
FILLMODE_RESPONSIVE - automatically sizes the canvas to fill its parent element using standard CSS dimensions.

This would greatly simplify responsive layouts, especially for apps with embedded or resizable UIs.

Implementation

This could be implemented with a minimal change here, adding a clause to handle the new mode using:

canvas.style.width = '100%';
canvas.style.height = '100%';

The Graphics Device would naturally get the correct dimensions during update via https://github.com/playcanvas/engine/blob/43d0ee2ffa6e0d49dd2350bb1b9f50b717fd2c65/src/framework/app-base.js#L1311-L1314

/cc @willeastcott @mvaligursky @Maksims