Abort the application on attempt to use any ES extension function.

We don't have extension loader for ES yet, thus we need to abort on
these to avoid undefined behavior. The only exception is NaCl, which
provides _some_ extensions without the need for extension loader. These
extensions are implemented in particular:

    CHROMIUM_map_sub
    EXT_occlusion_query_boolean
Text: disable test case if required extension is not available.
Doc++, added todos.
NV_read_{depth,depth_stencil} is not available in unextended ES3.
Disabling strict aliasing on GCC >= 4.8.

Causes failure of DebugToolsCylinderRendererTest without any warning,
only in release build, any attempt to add debug print results in issue
disappearing. Not an issue on Clang or GCC < 4.8.