fix problems of resizing

[chirsz-ever]

Let me explain the point. The resize opertion in demo is like that currently:

If the cursor's position may make the window's size too small, its size won't shrink more. That's all right. But the window's size enlarge as soon as the cursor move backward rather than keep its size until the cursor move out of the original press position, which windows in other GUI platforms perform.

I revise some code and this is the effect:

I add two members into mu_Context so that more than one mu_Context could work on the same process. the modifications does not destroy the library's lightweight and fixed-sized memory region.

[chirsz-ever]

In fact I am new hand at git and github, and I just learned how to use git rebase to simplify commits today.
It's a good experience to contribute to this great project :)

[chirsz-ever]

I rebased this branch to the latest commit of the master branch. The change should be easy to read and understand.
I apologize if I did offend or distrub you, please believe that is not my intention.
Lastly, If you insist on not to merge this PR, please close it.

[rxi]

Sorry for taking so long to get back to you on this -- I don't feel the functionality that this change provides is worth the added code complexity it adds, even if it is minor.

That being said, thanks for taking the time to contribute and putting in the effort to so clearly outline the changes with embedded gifs.