Improve thumbnail quality

[Meakk]

Describe the bug
Thumbnails are low resolution which make aliasing very noticeable, even with FXAA enabled.

Expected behavior
The image should be rendered at higher resolution and down-scaled to the expected output size.

System Information:

• OS: Windows/Linux

Additional context
Preferably, it could be done at rendering level by rendering in a larger framebuffer and blit the result to the final framebuffer (super-sampling) or done in the thumbnailers by resizing the result manually.