added missing import
block renderer now supports multiple textures

block rendering and block definition structure in chunkrenderer.c
changed to accomodate a texture per face instead of per block
definition.

blockdefinitions.py changed to match the new expected block definition
data structure.

blockdefinitions's make_box() function now correctly orients the NZ and
PX faces. before they were upside down.

Added a block definition for grass and for cactus. Cactus has a
non-standard model and doesn't use make_box().

chunkrenderer.c: added a buffer_clear function as an alternate to doing
buffer->length = 0.

changed block definitions image list to a set, to avoid accumulating a
very large list.
blockviewer: changed render angle

now all 6 sides of the block cube are visible