cogl-buffer: Track the last used bind target in CoglBuffer

This makes CoglBuffer track the last used bind target as a private
property. This is later used when binding a buffer to map instead of
always using the PIXEL_UNPACK target.

This also adds some additional sanity checks that code doesn't try to
nest binds to the same target or bind a buffer to multiple targets at
the same time.

cogl/cogl-pixel-array-private.h

@@ -58,7 +58,6 @@ struct _CoglPixelArray
 
   CoglPixelArrayFlags  flags;
 
-  GLenum                gl_target;
   CoglPixelFormat       format;
   unsigned int          width;
   unsigned int          height;