Merge Rect and Rectf classes.

[tylerandari13]

In honor of Supertux week, I have decided to contribute something of a decent size. This Pull Request merges a lot of int and float duplicates in Supertux into single classes with templates and using. These classes include:

• Rect and Rectf
• Size and Sizef
• Vector and Vectori (technically it was added, but still)

The float variants of all of the classes were prioritized because they are more up-to-date than the int versions. This means Rect now has some asserts that weren't there before, and it has caught a couple of oversights in a few places... aaand thats why its still a draft PR. I need to finish fixing a bug in the tilemap code. I am releasing this as a draft so that things like codestyle can start being checked.

[tylerandari13]

I forgot to mention in the description that I also replaced the left, down, up, and right properties with a function instead lol

[Hypernoot]

Size_t is not a good name, as it can be confused with std::size_t.