demos: update fbotexture.c to use EXT or ARB functions exclusively

demos: update fbotexture.c to use EXT or ARB functions exclusively

When the -arb option is specified we use GL_ARB_framebuffer_object intead
of GL_EXT_framebuffer_object.

For some vendors' OpenGL it's important to call the ARB entrypoints
instead of the EXT entrypoints to get correct behaviour.  Use some
function pointer tricks to do this (instead of GLEW).