block ID fixes + entity rendering fix + depth shading fix

* fixed some ID mismatches for new 1.8 doors
* added Piston Extension, renamed Piston Head
* rearranged alpha definition in block IDs to prevent future errors

* fixed X displacement rendering for entities

* fixed depth shading:
before this fix, semi-transparent blocks pop out much brighter than other blocks. You can test that behavior on a normal Plains Biome and switching on depth shading. Then Tall Grass or Flowers pop out much brighter than Grass blocks. When moving the depth slider these blocks doesn't change.
The new alpha blending is based on standard code like in the literature or used by OpenGL. As a side affect water will be rendered much darker, as before the top water block was also brightened up.