- Added NULL return if the replacement string (`by`) is empty
- Reordered sanity checks since there's no need to initialize variables if the strings are invalid.

* Fixed offset check in GenImageFontAtlas

* Fixed code formatting to follow raylib notation rules

Using new file I/O ABI

This issue was breaking multiple things...

Just for clarification, no plans to remove libc dependency, just did some code analysis to see how much raylib depend on stardard C library. My conclusions:

 - stdlib.h: primary dependency is for malloc() and free()
 - stdio.h: primary dependency is for FILE access, maybe it could go through a custom ABI?
 - string.h: just around 8 functions required
 - math.h: just around 8 functions required
 - others: 1-2 functions required for some other headers

Added SUPPORT_TRACELOG_DEBUG config

This could be required on texture packing ;)

- Renamed GetGlyphIndex() parameter
 - Review DrawTextEx() implementation
 - Review DrawTextRecEx() implementation

Just simplified code a bit

* explained a bit more the core_window_letterbox example

* fixed a few 'ups' moments that could lead to mild head pain and time loss

ADDED: CodepointToUtf8()

RENAMED: TextCountCodepoints() -> GetCodepointsCount()

- Improved LoadBMFont()

Now it accepts a ';' separated list of extensions, useful to check multiple extensions

Get the unicode equivalent characters (as int array) from a UTF-8 text array... maybe this function is renamed to be clearer to users...

Some generated fonts were not fitting the image...