Added struct Vector4 for convenience

When UnloadModel() --> UnloadMaterial(), avoid unloading default shader (if used) and avoid unlaoding default texture (if used), that data is managed by raylib internally. The question is... should UnloadModel() also UnloadMaterial()?

Apparently, before macOS Sierra version, clock_gettime was not available, using MATCH timming system instead

Supported UTF8 range equivalent to [128..255] (80h..FFh)
Exposed and renamed text function GetGlyphIndex()
Renamed spriteFont parameter name to simply font
Small security check on transmission mission ending screen

They were named so for compatibility with make, but make doesn't use
the anymore. I always forget whether it's SHARED_RAYLIB or
RAYLIB_SHARED...

For now, RAYLIB_SHARED and STATIC_RAYLIB may still be used,
but print a deprecation warning.

Rename: GetHexValue() renamed to ColorToInt()
Added: ColorToHSV()

These changes are intended to provide context and control over the Linux make install process. make install RAYLIB_LIBTYPE=SHARED now works as expected. libraylib.so is properly installed with attendant runtime links. The examples will be running against the newly installed libraylib.so unless otherwise specified with RAYLIB_RUNTIME_PATH. See raylib/src/Makefile and raylib/examples/Makefile for usage of RAYLIB_INSTALL_PATH, RAYLIB_H_INSTALL_PATH, and RAYLIB_RUNTIME_PATH variables. RAYLIB_RUNTIME_PATH in particular is interesting for portability since sudo isn't needed.
The default configuration and workflow is essentially unchanged and unaffected.

If user doesn't build the static library,
`pkg-config --static --libs raylib` should be equivalent to
`pkg-config --libs raylib`.

Fixes this build failure:
http://www.cpantesters.org/cpan/report/a069fade-0e1f-11e8-a1cf-bb670eaac09d

- Some functions renamed
- Comments reviewed
- Functions reorganized

Otherwise we may run into LoadDefaultFont and crash in rlLoadTexture
Also moves InitTimer() before InitGraphicsDevice(), to allow it to be
tested even if InitWindow ultimately fails.

Otherwise we run into an assertion failure inside GLFW's glfwGetVideoMode.
Example:
http://www.cpantesters.org/cpan/report/b4ba5894-0bdb-11e8-841e-2c60b04e1d2d

This is related to #456.

Allows loading of shader from text code string directly

Note that this version is under development and could be buggy on some
platforms...

In case graphic device could not be created it returns false instead of
failing with an error tracelog (and consequently closing the program).
Window initialization success could be checked with new function
IsWindowReady()

You can't do much with raylib if glfwInit or glfwCreateWindow fails,
currently it just exits by means of TraceLog(LOG_ERROR.
User code, however, might want to fall back to a text-only UI
or display a warning if raylib can't be used.

glfwSetWindowPos was called on a NULL window, triggering an assert
inside GLFW. Check for failure and exit cleanly by means of
TraceLog(LOG_ERROR instead.

-DWITH_SYSTEM_GLFW=ON: Link against system glfw and fail otherwise
-DWITH_SYSTEM_GLFW=OFF: Use embedded rglfw.c
-DWITH_SYSTEM_GLFW=IF_POSSIBLE: Probe for system glfw but fallback to
                                rglfw if unavailable

Also change Linux 64-bit CI build to install system glfw and use it,
so this doesn't bitrot.

Addresses #453.

Cherry-pick from upstream glfw/glfw#1196
Found in CPANtesters test of Alien::raylib:
http://www.cpantesters.org/cpan/report/d956d128-0339-11e8-b0d1-b6c4abd39192

Just in case ClearBackground() is not used after BeginTextureMode(), it clears fbo to color defined by previous ClearBackground()

which is the default behavior on C99 and up.

When changing image format, mipmaps are lost and regenerated from
scratch

Review rlLoadTexture() function to make it simpler, now OpenGL texture
glInternalFormat, glFormat and glType are retrieved with new function
GetGlFormats()

As an extension, GNU C treats sizeof(void) as 1. MSVC doesn't.
Make it an error on GCC/Clang to avoid accidental MSVC breakage.

GNU C allows it as a compiler extension, but MSVC doesn't.