RENAME: GetLastWriteTime() to GetFileModTime()

Added GetLastWriteTime to allow for file reloading

- 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.

Fixes memory leak in text.c

3 minor sample game cleanups:

Physac.h fix for variable array size declaration.

GenImageFontAtlas prototype in raylib.h is incorrect.

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.

* changed bitwise & to logical &&
* removed redundant state check
* moved a strcpy inside a NULL check to prevent a crash.
  NOTE: nothing sensible will happen as a result, but no crash at least.

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.

core: workaround window not being rendered till moved on macOS Mojave

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.

[physac] Fix physac's fixed time step

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

Added monitor functions

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

- Added else so return not compiled twice.