Minor fixes to custom_shader example

[mfreeborn]

A handful of minor changes and fixes.

1. Vertically align the controls with their labels
2. Correctly use rads when defining the camera's field of view, rather than degrees (which does happen to work fine, but is not what is intended. The original code looks to be taken from the learn WGPU tutorial, which uses cgmath, rather than glam, which, in turn, uses Rad and Deg types.)
3. The dynamically resizable Buffer never does updates its own size value upon ::resize(). It looks like we can instead rely on the underlying raw buffer's ::size() when determining whether the raw buffer should be recreated. The pre-existing behaviour recreates the raw buffer every frame, because size is initialised to hold just one cube, and thus the condition to create a new buffer is basically always met.