Enabled by default, makes the current Magnum context a thread-local
variable instead of a global one, so it's possible to have multiple
thread-local contexts. Might have some performance implications, that's
why it's possible to disable it (but enabled by default is the safer
option).
GCC 4.7 and Apple platforms don't support thread_local, but __thread
does the job too (though on iOS not until Xcode 7.3). Also had to move
it to file-local because MSVC doesn't like having thread local variables
as part of DLL interface. (And there is *of course* no way to disable
exporting one particular member. F' that.)
What's done:
* The build matrix now has four targets -- Linux desktop, OSX desktop,
iOS GLES2 and iOS GLES3.
* Putting the commands into separate bash scripts, which are called
based on what's the actual build matrix item.
* Displaying *all* test output (colored) for visual verification.