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.

InitWindow: return false if no monitor found

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

Bump version to 1.9.4

While nice to test, this would mean that the tagged release will depend
on GLFW as well… Therefore disable it for now.

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()

for consistency with Makefile. Requested by @raysan5 in #453.

Return false from InitWindow if glfwInit or glfwCreateWindow fails -TO REVIEW AS DISCUSSED-

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.

Let it be noted I utterly dislike their syntax.

-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