Screen/OpenGL-ES: Make floating point textures and framebuffers work.

Turns out they work just as on desktop GL, so remove some of the
special casing. Now we have full support for 32 bpc floating point
textures and framebuffers, possibly with hw accelerated bilinear
texture filtering/sampling and alpha blending.

This is tested on NVidia binary blob with X11/GLX + OpenGL-ES-1.
All shaderless floating point texture/framebuffer demos seem to
work well now.

We keep 16 bpc floating point formats disabled - with silent auto-
upgrade to 32 bpc. This does not seem to be strictly needed, but
it simplifies things a little bit.

Apart from what is apparently a NVidia driver bug for RGBA8 textures,
this version of the code works well on NVidia proprietary drivers under
OpenGL-ES1.x

-> Screen.mex for 64-Bit Octave on Linux rebuilt and tested.

=> This defines a good base-line and starting point for actual
   porting to the embedded/mobile Nexus-7 test device.