Let the user choose if using rres external library

Moved function DrawRectangleT() from text module to shapes
Added some comments

Usefult to draw rectangles using default font texture

...to avoid possible conflicting symbols

LoadSpriteFontTTF() --> LoadSpriteFontEx()

enum LogType could require some revision...

Replaced old GetExtension() function
Make IsFileExtension() public to the API

It seems Sleep() behaves weird on my computer, disabled by default
returning to the busy wait loop... also re-implemented DrawFPS() to
avoid frame blitting...

Working in modules configuration flags...

Now it uses CharInfo data, this way, it's better aligned with the future
RRES file format data layout for sprite font characters.

Now program is halted (OS signal call) for required amount of time every
frame, so CPU usage drops to zero, instead of using a busy wait loop.

- Renamed WritePNG() to SavePNG() for consistency with other file
loading functions
- Renamed WriteBitmap() to SaveBMP() for consistency with other file
loading functions
- Redesigned SaveBMP() to use stb_image_write

Mask was wrongly applied to 8-bit font image, it generated dark borders
on the font. Grayscale image has to be considered as the alpha mask for
a completely white image to use it correctly.

Still not working correctly, font offsets are not considered
correctly...

Support by default unordered charsets

Still requires some testing...

LoadSpriteFontTTF() - TTF font loading with custom parameters

Support grayscale (8 bit) textures for fonts
Load unordered chars data above char 126

Useful if text module is not required...

raylib uses spaces between '+' and '-' signs but not between '*' and '/'
signs, it's a chosen convention

Moved all external libraries used by raylib to external folder inside
raylib src. Makefile has already been update and also the different
includes in raylib modules.

This is a first step toward a bigger project. Some modules could be
ported to header-only to be used as standalone.

- Check if first character is the expected Space char (32)
- Check if characters are ordered in definition file (.fnt)