-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.

- 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...

- 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

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

Just found I need that function...

Using default font texture as base white texture for rectangles reduces
draw calls considerably, actually, raygui can be drawn with a single
pass!

Issues when dealing with FBOs

Review comments

Functionality integrated in DrawRectangle() and selectable with config
flag USE_DEFAULT_FONT_TEXTURE

Support external GLFW usage
Renamed some flags for consistency

I think is clearer this way...

Trace log messages could be configured with this function to select wich
ones are shown

That function depends on SavePNG() and forces platform check on textures
module, that behaviour should be changed, platform definition should be
only required on core module...