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.

...and MacOSX OBJC types definition...

Expose native Cocoa Window again...

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

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.

- 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.

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

NOTE: These changes break the API (parameters order)

Some minor tweaks around

strprbrk() does not follow raylib function name convention but as it is
very similar to the standard strpbrk(), we'll keep the naming... also,
don't like to mix static functions with API functions but as strprbrk()
is just used in the two functions below, it's ok to be there... maybe it
could be refactored in a future or even move the logic inside the
required functions...

Noted in #634.

Fixes #634.

PLATFORM_WEB is not the only platform that doesn't define
GLFW_TRANSPARENT_FRAMEBUFFER, it's also recent enough that it's not
included in the most recent GLFW 3.2.1 release, so this error can creep
up when using USE_EXTERNAL_GLFW.

Examples can be compiled for web with no code change at all! Usually
examples need to be refactored for web... using emscripten code
interpreter (emterpreter), it can manage synchronous while() loops
internally... as a downside, execution is very slow...

This change allows rlgl.h usage as independent single-file header-only module... still some tweaks required, like removing GLAD dependency... required extensions could be manually loaded!

Also removed shader_distortion.h, embedded in rlgl.h

Joystick support is not implemented just yet, not difficult but it
requires some work...

...by emscripten glfw javascript implementation

Try to avoid types conflict with Font

- Renamed Begin3dMode() --> BeginMode3D()
- Renamed Begin2dMode() --> BeginMode2D()
- Renamed End3dMode() --> EndMode3D()
- Renamed End2dMode() --> EndMode2D()

Allow compiling the library with support for automatic screen capture
(KEY_F12)