Added NinePatch struc definition and function prototype.

The GLFW tree distributed with raylib has two modifications:

- GLFW_PKG_{DEPS,LIBS} are exported to PARENT_SCOPE, so we can use them
  in our pkg-config file
- An intermediary glfw_objlib target is added, so we can reexport GLFW
  symbols from libraylib.a

rglfw can fix the second point, but for Wayland usage, we would have to
replicate protocol generation, so we just leverage GLFW's existing
support instead.

To make maintenance easier, I have submitted a pull request for
including these modifications to upstream GLFW.
And to make that one easier, this patch dog-foods the modifications,
so raylib users can help find regressions. :-)

glfw/glfw#1307

So user code can use add_subdirectory to build it (similar to what we do
with GLFW or what the projects/CMake/CMakeLists.txt can do).

The CMakeLists.txt checks for an installed raylib and downloads and
installs one if none is found. Afterwards, it builds core_basic_window.c

Homebrew doesn't ship 32-bit libraries anymore, so using both
-DUSE_EXTERNAL_GLFW and -DMACOS_FATLIB with a Homebrew GLFW would fail.
With -DUSE_EXTERNAL_GLFW=OFF, this is not a problem, but I think keeping
it off by default makes more sense. If someone wants universal
libraries, they can just toggle it.

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.

Remove that link_libraries_to_executable() hack and defines a proper
raylib target that can be used with target_link_libraries.

The same target is also available for external (user) code by using
find_package(raylib).

This results in:

- Remove hardcoded build directories from examples and games CMakeLists.txt
- Allow rlgl_standalone and other special examples to be built easily
- Allow CMake projects to find_package(raylib instead of fiddling with pkg-config
- Makes code a little more maintainable
- Fixes #471, #606.
- Makes code less confusing by removing the double use of PLATFORM (#584).

Note that this is still not _The Right Way_(TM), because normally
raylib-config.cmake (or its includes) would be automatically generated.
I didn't manage to get that to work though, so I went the easier route
of just wrapping pkg_check_modules for consumption by find_package.

also makes one configuration shared-only

dr_flac is not built on i386, because GCC 4.8 complains about asm("cpuid"
clobbering ebx, as it's using ebx for PIC.

Instead of downloading a newer GCC (and multilib),
disable FLAC for that one configuration...

free() and NULL at the end.

It would be good to have a way to continue execution even when loading a stream didn't work.
 Free music and set it to NULL in case something went wrong, so that users can check for it.

Still a work in progress but it already works...
Current riqm API could be simplified...

as well as -DUSE_EXTERNAL_GLFW.

Nips issues like #584 in the bud.

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

Corrects issue with audio on RPI

pkg-config --libs --static raylib and pkg-config --libs raylib
should give the same result if there is no shared raylib installed.

Vector4 -> Quaternion
Texture2D -> Texture
RenderTexture2D -> RenderTexture
Camera3D -> Camera
SpriteFont -> Font

Fixes: #568

Note for me: Replace All is NOT your friend...

Library consistency rename... yes, I know, it breaks the API...

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