ImageDraw() code tweak

Reduced draw calls

Added note idea on ImageFormat() for future redesign (to support
16bit-32bit pixel-formats)

Not used in raylib this mechanism

Replaced by fabs() that seem to work ok

It seems TCC was not casting correctly int values to float in some
specific situations

Added android_native_app_glue module to raylib compilation instead of
compiling it as static lib at apk generation.

Temporary measure to fix a breakage on FreeBSD.
See #551 for more details.

Did this ever work? Surely, doesn't look like it...

I don't know if this actually works, but it compiles, which is good enough for CI.

src/core.c does explicitly skip including <camera.h> on Android
and tests fail. Therefore skip these failing tests.

Closes #507.

Not sure if this ever worked, but now it at least compiles.

Release, unless we are in a Git repo, then it's Debug.

From 2019 64bit support will be mandatory to publish an Android app.
Google plans to require that new apps target Oreo (API level 26) in
August of 2018.

On OpenGL 1.1 only is supported for 32bit RGBA textures

Increase API version

Please do this whenever necessary. And adapt it before releasing a new
RC. The RC should already have the correct one.

Now defaults to Clang, ARM64 and API 21

This reverts commit 2d6fb5c628068757387525e190c3afdbe33ae9c6,
and adds a fix for Alien::raylib's test failures.

The tests failed because the resulting static library didn't reexport
GLFW symbols. As a fix, we now have GLFW create a CMake "object library"
target that we can link with both the static and shared raylib.

This is arguably ugly... Proper fix would probably be a GLFW upstream
object library target.

Closes #536.