Verify support for various general state before restoring.

Further checks for both GL_ARB_imaging and GL_ARB_shader_image_load_store extension support prior to restoring related state.  These extensions not currently supported on Intel MESA.
Only query FBO params for "LAYERED" texture if GS extension supported

Currently not supported on Intel MESA.  Enum "GL_FRAMEBUFFER_ATTACHMENT_LAYERED" was introduced by GL_ARB_geometry_shader4 extension so dynamic check for this support prior to query seems reasonable.
Modify assert on invalid displaylist to a warning.

This assert occurs on Intel/MESA but doesn't need to be fatal.
Merge branch 'master' of https://github.com/ValveSoftware/vogl
Merge branch 'master' of https://github.com/ValveSoftware/vogl
Merge branch 'master' of https://github.com/ValveSoftware/vogl