Enable FBO support; fix a couple bugs.

Enable framebuffer object support.  I'm starting to see the value of
GLEW, but the implementations available don't handle GLES very well.  A
primitive attempt at cross-platform compatibility at the source code
level was attempted, but the Correct Solution is runtime adaptation.

Fixed a bug in PCM.cpp; it now properly scales int16_t values to the
range -1.0 - 1.0.

Fixed a bug in Renderable.cpp; 'size' can be zero, leading to an attempt
to allocate and render a buffer of zero size.  While the code correctly
handles zero-sized buffers, it's better to avoid it entirely.
6 files changed