Visual rescaler added

Visual rescaler added to main branch. Be careful: it uses memory for preview, which increases with every run; don't try to use it on very large images.

Bug with arbitrary caps in extensions fixed. Wasn't discovered and fixed earlier because nobody seem to use such an extensions in real life.

As usual, for Windows users compiled ScaleNx .exe files are available:

Windows EXE	Tested Windows version
ScaleNxGUI64.exe	Windows 10 64-bit
ScaleNxGUI32.exe	Windows XP 32-bit