Expose `resize` method for regl-managed contexts

[joepie91]

There's a resize method that gets called internally whenever the window resizes; however, it's not accessible from the consuming code.

Sometimes other factors can cause a container element to become resized, like when using a layout management library such as FlexLayout.

It would be greatly helpful if, at least for cases where regl manages the context and canvas, a resize method were exposed on the regl object (eg. as regl.forceResize), such that it's possible to manually force a resize operation when needed.

[danman113]

Just going to bump this because I think it's a good idea

[illus0r]

There is a workaround described in #431