Makefile and vcxproj file updates.
Also modified vcxproj files so that the various files ispc generates go into $(TargetDir),
not the current directory.
Modified the ray tracer example to not have uniform short-vector types in its app-visible
datatypes (these are laid out differently on SSE vs AVX); there was an existing lurking
bug in the way this was done before.
This applies a floating-point scale factor to the image resolution;
it's useful for experiments with many-core systems where the
base image resolution may not give enough work for good load-balancing
with tasks.
