Fixes to get default FB snapshot working on Intel w/ MESA

Use XGetWindowAttributes to get window size in place of glXQueryDrawable.
Mesa glXQueryDrawable has trouble reading width and height.

Use OpenGL calls instead of glX calls for vogl_get_default_framebuffer_attribs.
That will be portable to systems not using glX.

Modify order of linking vogleditor so that X11 comes last.
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.
Merge pull request #96 from jon-lunarg/github_07_11

Various fixes from Lunarg folks to get Intel/Mesa working better with state snapshot save and restore. Additionally, the "Set GL func ptrs patch ..." patch appears to be needed on Linux generally and is not Intel/mesa specific.