-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.

Game developed in the GGJ18, it has some bugs and ending screen is
incomplete... I'll try to keep working a bit more on it.

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

Removed by mistake.

Noted by @eserte in athreef/Alien-raylib#2. Thanks!

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

Make function calls without prior declaration an error

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()

Reviewed VS2015 UWP project

Integration of UWP support

GCC/Clang: Treat void pointer arithmetic as error

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.

- Support OpenGL 1.1 if selected macro
- Corrected MSVC compiler issues with (void *) data
- Removed raylib.dll project
- Created DEBUG_DLL and RELEASE_DLL configurations

Added support for noise image offset

- Added support for mipmap levels loading
- Removed internal function LoadTextureCompressed(), not required any
more...

Not required any more!

- Renamed VERSION to RAYLIB_VERSION
- Renamed API_VERSION to RAYLIB_API_VERSION
- Renamed RAYLIB_RELEASE to RAYLIB_RELEASE_PATH
- Support Web Assembly compilation on PLATFORM_WEB

Add mouse input and revert keyboard to callback model

Mouse input is implemented, with all bells-and-whistles. This includes
cursor locking and scroll wheel support.

Keyboard input is reverted to a callback model to better reflect the
existing architecture in "core.c"

Matrix stack system should be reviewed but, in the meantime,
currentMatrix should be reseted in order of 3d to work

If not using busy wait loop, linkage with WINMM in Windows platform is
required

Note that this solution is a temporal patch, not properly tested and
prone to fail, specially if matrix are pushed into the stack...

- Added: ImageAlphaClear()
- Added: ImageAlphaPremultiply()
- Reorganized some functions

- Renamed enum TextureFormat to PixelFormat for consistency
- Added support for pixel format UNCOMPRESSED_R32
- Using GetPixelDataSize() where required