SDL2 | Use best scaling method depending on resolution

[midwan]

If it's possible to scale up the resolution as an exact multiple (emulator vs native resolutions), we should use nearest neighbor which provides a sharp picture.
In other cases "linear" should be used to avoid an ugly distorted image.

Example:

• PAL modes scale up to 1280x1024 exactly (320x256, 640x512, 1280x1024 are exactly multiples) and if the native resolution of the monitor is such, we can easily use "nearest neighbor" scaling without problems.

• Picasso96 modes such as 800x480 on a 1080p native resolution, should not use "nearest neighbor" as it would give an ugly result - instead, "linear" scaling works best in those conditions.

Suggestion:
Implement an mechanism to calculate requested vs native resolution and switch scaling method automatically, in order to provide the best image output.