[examples] Added core_viewport_scaling

[nezvers]

Created an example for scaling a viewport RenderTexture in different modes:
Upscaling for pixel art and Lowrezjams

• Keep aspect integer
• Keep height integer
• Keep width integer
  Upscale and downscale (4K example included)
• Keep aspect
• Keep height
• Keep width

Example include couple preset viewport resolutions.
Modes and resolutions are switchable with gui buttons.
For example I included position on texture calculation with a circle following mouse.

I couldn't figure out how to use rexm yet, I'll try to get it working on next example. I did example adding manually without couple last checkpoints.

[raysan5]

@nezvers Thanks! Looks great! Please, could you review code to follow raylib code conventions?

Also, code structure should follow this format: https://github.com/raysan5/raylib/blob/master/examples/text/text_unicode_ranges.c

With the section headers, functions declared before main() but defined after main, static symbols...