Condition state capture and restore of state related to ARB_imaging extension.

Some drivers (such as Mesa) do not support GL_ARB_imaging extension. Make the
state capture and restore of state added by this extension be conditional based
on driver support of the extension.
Only save GL_TEXTURE_BUFFER objects when they are supported

Detect if GL_TEXTURE_BUFFER objects are supported by the driver and only save them in the case when they are.  Intel MESA doesn't currently support them so this prevents assertions.
Set GL func pts prior to restoring from snapshot at start of trace.

Without this we're hitting segF on Linux when running from a trace that starts by restoring from a state snapshot.