Fixed|Weapon Info: "Static" psprites (weapon offset scale)

When drawing psprites, the weapon offset scale that is used for
preventing the lowering of the psprite was not applied as intended:
WEAPONTOP cannot be ignored as that is the highest offset that the
psprite will reach (non-zero).

Todo for later: Make WEAPONTOP a DD_value so that it doesn't have to
hardcoded.
libdeng2: Added an operator+= for Block with IByteArray argument
libgui: Added a header for OpenGL (ES) 2

Includes all the needed headers in order to use OpenGL (ES) 2 on the
current platform.
libdeng2: Added Asset, a generic declaration of an asset's state

"Asset" means any kind of resource or other required data that is
needed by someone.

This will be used with Widgets at first.