Fix: Motion blur should sample onscreen fragments with no depth

[aevyrie]

Objective

• Motion blur does not currently work with skyboxes or anything else that does not write to depth.

Solution

• When computing blur, include fragments with no depth, as long as they are onscreen.

Testing

• Tested with the examples - the motion_blur example uncovered a bug with this fix, where offscreen pixels where now being sampled and causing artifacts.
• Attached example showing the skybox being sampled in the blur (note the feathering on edges):

Screen.Recording.2024-05-30.at.1.34.55.AM.mp4