Users can define their custom memory management macros.

NOTE: Most external libraries support custom macros in the same way, raylib should redefine those macros to raylib ones, to unify custom memory loading. That redefinition is only implemented as example for stb_image.h in [textures] module.

Custom implementation that returns -1 if it fails (no negative values supported)

Getting the following strict error....

```
src/text.c:117:105: error: constant expression evaluates to 2398692640 which
      cannot be narrowed to type 'int' [-Wc++11-narrowing]
...0x00000000, 0x00000000, 0x00200020, 0x0001b000, 0x00000000, 0x00000000, 0x8ef92520, 0x00020a00...
                                                                             ^~~~~~~~~~
```

Switching it to an unsigned int fixes it.

Considering chars inputs come in UTF8 form!

Added a bunch of useful text management functions.
Consequently, some already available functions like `FormatText()` and `SubText()` has been renamed for consistency. Created temporal fallbacks for old names.
raylib version bumped to 2.3.

Changed parameters order for consistency with LoadFontData() and other functions when an array is passed by parameter and array size is the following parameter.

Useful in case raylib compilation want to be automated and compilation config flags provided by command line.

stb_truetype supports some .otf fonts, just exposed that functionality.

text:     IsEqualText() -WIP-
audio:  SaveWAV()
audio:  ExportWaveAsCode()
textures:  ExportImageAsCode()

GenImageFontAtlas() allocates an array of stbrp_rect for the packing functions, but it never frees them.

REVIEWED: ExportImage()
REVIEWED: ExportMesh()
ADDED: ExportWave()
REMOVED: Internal funcs: SavePNG(), SaveBMP()

NOTE: These changes break the API (parameters order)

Some minor tweaks around

Useful for bitmaps pixelated fonts where anti-aliasing is not desired!
Change also enables additional font generation mechanisms in a future (cleartype, hinting...).

Note for me: Replace All is NOT your friend...

Library consistency rename... yes, I know, it breaks the API...