Move platform macros from sysutil.h to platform.h
Add OIIO_FORCEINLINE definition
Simplify includes
Refactor IBA -- break up unwieldy imagebufalgo.cpp.
Move all high-quality filtered transformations/warps to imagebufalgo_xform.cpp.
Move all drawing and visible annotation to imagebufalgo_draw.cpp.

No actual code or functionality has changed, just moving things from file
to file for better organization and faster recompilation.