Provide preliminary support for OpenGL-ES2 shadow mapping

Split shader initialization for regular and cube shadow maps,
detect GL_OES_depth_texure extension for OpenGL-ES 2.0 for regular
shadow maps. Most architectures support this extension.

This is work in progress and has not yet been tested. Although
the current shadow map support in the shaders (which is not
very optimized) may work, optimized shadow map support may require
more use of built-in functions (which could help OpenGL 3+ as well).

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>