Fixed|XG|libcommon: Misuse of dummy mobj in XG leading to a crash

p_xgsec was converted to C++ with some minimal cleanup. XG now allocates
thinkers using ThinkerT, and does not call memset on the dummy mobj
(which is a C++ instance).
Fixed|libdoomsday|Thinker: Crash at shutdown after zapping a thinker

Zapping should retain the malloc flag of a thinker so that it can
be later freed using the appropriate function.
libcore|Variable: Added an automatic cast to non-const Record
libcore|ScriptedInfo: Observing newly parsed block records
libcore|Bank|InfoBank: Multiple source files combined to a single namespace

InfoBank now has a single namespace where all the parsed sources will
be combined. Previously the earlier Info contents would have been
discarded when a new source was parsed, leaving only the items added
to the bank.

Now the combined namespace has all the contents from all source files.
A special "__notInBank__" member is temporarily added to each block
to detect which items have already been added.
libgui|ModelDrawable: Audience for impending GLInit

Third parties may want to perform additional setup right before a
model is initialized for drawing.
Model Renderer|GL: Shader defines texture mapping for itself

The shader definition can specify which texture map is to be bound
to which position in the vertex attributes. The model renderer checks
this information when a model is being prepared for drawing.