Incorporate viewport position into clamping in camera_system

[mweatherley]

Objective

In its existing form, the clamping that's done in camera_system doesn't work well when the physical_position of the associated viewport is nonzero. In such cases, it may produce invalid viewport rectangles (i.e. not lying inside the render target), which may result in crashes during the render pass.

The goal of this PR is to eliminate this possibility by making the clamping behavior always result in a valid viewport rectangle when possible.

Solution

Incorporate the physical_position information into the clamping behavior. In particular, always cut off enough so that it's contained in the render target rather than clamping it to the same dimensions as the target itself. In weirder situations, still try to produce a valid viewport rectangle to avoid crashes.

Testing

Tested these changes on my work branch where I encountered the crash.