fix loading vegetation

[kirides]

• grass resource Open-check was inverted. fixes No grass G1 #424
• Grass is now also producing shadows
• GVegetationBox now shares the engine resources (mesh & texture) instead of loading them per-instance.
• GVegetationBox no longer stores a (per frame!) constant buffer per instance. Instead we grab a free slot from out dynamic materialinfo pool buffer.
• Grass wind effects can now be disabled using the regular "Wind effects" toggle like other vegetation.
• Grass: Hero affects objects now also works for grass