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

-DBUILD_SHARED_LIBS=OFF == -DSHARED=OFF -DSTATIC=ON
-DBUILD_SHARED_LIBS=ON  == -DSHARED=ON  -DSTATIC=OFF

Fixes #626.

Apple ought to fix their OpenGL implementation, but with OpenGL now
deprecated this might not happen.

This has been reported upstream in GLFW in glfw/glfw#1334.
The workaround comes from kovidgoyal/kitty@b82e74f99

This also fixes the HiDPI (Retina) scaling issues reported in #497,
so the workaround is enabled anywhere __APPLE__ is defined.

Previously, if <raylib.h> was #included prior to another header that
defined bool, the compilation would fail.
This is e.g. the case for <perl.h> and <objc/objc.h> which both fall
back to the <stdbool.h> if C99 is available.

The following commit includes <objc/objc.h> in src/core.c, which causes
the same problem.

Avoid this by checking for C99 bool like we already do for C++'s.

A single warning at configuration time is enough.

[textures] Added SaveKTX()
[rlgl] Exposed rlGetGlTextureFormats()

- Using same idea as SetWindowMonitor to take in a index with 0 being the primary monitor.

- Added else so return not compiled twice.

- Added PLATFORM_DESKTOP check for Monitor functions to try to fix issue on android.
- Not sure what return types should be when not on desktop. Added rough guess for now.

- Fixed tabs used instead of 4 spaces

- Get number of monitors
- Get size, physical size and name of primary monitor. Could pass monitor id instead not sure.

Added note on KTX 2.2

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

NOTE: These changes break the API (parameters order)

Some minor tweaks around