Fixes for latest version of cmake + clang.
Now successfully compiles using SDL release 2.0.0

Previously, we required SDL HEAD, for the highdpi rendering code.  Now,
cmake checks whether if SDL library supports high dpi rendering, and
cleanly drops VectorStorm support for high dpi if not.
Removes vsOverlay from VectorStorm.

We haven't supported overlays since VectorStorm was a 2D engine.  They
were a quirky, fun idea.  But not actually performant or useful for real
games.  And they haven't worked in years, anyhow, so I'm finally
removing their old source files and the code which used them.
Fixes stencil rendering for bloom/shader render schemes.

Offscreen render buffers weren't having their stencil buffers cleared.
Oops.
Cleanups to bloom rendering.

Mostly removing old, commented-out code.
Fixes a compile error in vsArrayStore, under Visual Studio

(In templated code which isn't actually being called from anywhere, so GCC
wasn't actually compiling it.  But Visual Studio compiles it anyway)
Commenting out img_savepng.c

Shouldn't be necessary any more.  This is a first step toward
removing it entirely.
Move the generated VS_Config.h header to a separate directory.

(rather than sitting straight in the build directory)
Fixes for Windows SDL2 builds.
Fixes screenshot functions for HighDPI displays

(Previously, was only screenshotting a small portion of HighDPI
displays)
Try to push TCP network data immediately.

(Previously, would try to queue it up, even if nothing was busy on the
socket already).
Switches vsImage::BakePNG() to use IMG_SavePNG_RW()

Previously, was using libpng where available.
Adds a 'vsync' rendering preference.

Also removes a test for glDrawElements -- that's always present;
everywhere from OpenGL 1.1 to OpenGL 4.x;  since we test for OpenGL 2.1
support, we should be fine.  (And we couldn't cope with not having it,
anyhow)