Make osd a cmake object library.

An object library allows other build targets to use the object files
from this library.

The change introduces osd_static_cpu_obj which is consumed by
osd_static_cpu.

This will be useful for emscripten integration where we cant use the
compiled library, rather it will use the object files, targeting
osd_static_cpu_obj.
Remove CMake compiling redundancies

- set OBJECT targets for osd cpu & gpu libs, and use the obj target for
  static and dynamic linking

- add a new examples_common_obj OBJECT target
- replace direct source dependencies to obj target in all examples CMakeLists

This change makes it possible to not re-compile the same source files
multiple times when they are used in multiple targets. Thanks to jcowles
for uncovering the CMake functionality.

Note: it seems that multi-process build is working again (gmake -j <x>)