Wait for input events passively (sleep) instead of polling events actively every frame

- Updated dr_mp3 and implemented it
- Reviewed sampleCount vs frameCount
- Reviewed XM playing (some weird things...)

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

The comment sais it allows max 256 files, but the MAX_DIRECTORY_FILES
define actually is set to 512.

Let's have a seperate if linux block for this. Since we will need to
define MAX_FILEPATH_LENGTH for all other cases. And its more readable
like this.

On Linux systems the length of the path is defined in linux/limits.h as
PATH_MAX.
Lets use that for MAX_FILEPATH_LENGTH, which is a define that we can
find in some comments in the code that is actually never used. Instead
often we see 256 handwritten.
So lets have MAX_FILEPATH_LENGTH as a proper define, being set to
PATH_MAX on Linux and to 256 on Windows systems.

Why am I still supporting this???

...disposing RenderTexture.

Loading full mp3 file works but loading for music streaming does not work, it seems total number of samples is not obtained correctly from mp3 file...

RENAME: GetLastWriteTime() to GetFileModTime()

- Added a function to get the last write time of a file. I used this so I can reload files or resources if the time since they were last loaded changes.

OPENGL_VERSION is the version requested by the user,
not the detected one...

Fixes #660.

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

The prototype of GenImageFontAtlas() in the main raylib.h header has a set of swapped parameter names.
Going through the usage of the function within raylib itself, it appears as if the correct order is:
1) charsCount
2) fontSize

However, it's exposed to the world as the other way around, which may cause some major confusion.

Generating the projects using CMake, targeting VS2017, results in an error when compiling.
This is due to physac.h trying to make a 'vertices' array of size 'int count', making it const does not work, either.

This changes the static declaration to a malloc/free combo.

Tested using the physics-demo.

...and MacOSX OBJC types definition...

Expose native Cocoa Window again...

This implementation is a bit limited, does not support wide characters directories but it's a temporal solution...

This solution uses "io.h" (like MinGW provided one) while other modern solutions use the "windows.h", that result in duplicate symbols issues... need to review it carefully.

Could not be retrieved for now...

Not supported for the moment, issues with Linux (symbol `Font` redefined) and OSX (NSGL type redefined)

REVIEW: Reorganized global variables for consistency
ADDED: GetWindowHandle() to get native window handle
ADDED: GetDirectoryFiles() to get files list for a DIR