Allow user to specify the viewport resolution

[yhslai]

Is your feature request related to a problem? Please describe.
Sometimes I'd like to check how my shaders look like in a square resolution, or a portrait one. At this point there isn't an easy way to do this. I can view it in 800x450 (the default), the fullscreen resolution, divided by 2, etc. But there isn't a way to make it render at a specific resolution (e.g. 1024x1024, 512x1024, etc)

Describe the solution you'd like
Add two input fields to specify resolution of the canvas. For example 1024, 1024.

Describe alternatives you've considered
Haven't found a solution to this.

Additional context
Add any other context or screenshots about the feature request here.