Demo: fixed resizing while maintaining aspect ratio constraint

[imdongye]

before version problem:

1. can't resize vertially with grap corner.
2. can't resize with grap top and bottom edges.
3. there is no status value to check which part of the window (corner, edge, etc.) is being used to resize the current window.

my sollution is using mouse cursor status and aspect ratio of next window size

after version:

usage my framebuffer viewport code

[ocornut]

Linking to #6210