Add variable block hardness. Closes GH-17

[satoshinm]

#17 Block-breaking time with block hardness and breaking animation (vs instamine)

[satoshinm]

This basically works, but a visual block breaking progress indicator is necessary to let the player know there is progress being made and the game is not merely lagging (#100 Lag when mining/building many blocks repeatedly?)

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To indicate the progress, should display an ever-increasing texture over the block textures. This only needs a textured quad, similar to text rendering. Or more to plant rendering? gen_plant_buffer() calls gen_faces(10, 4), two quads crosshatched:

(not billboards, tutorial: http://www.opengl-tutorial.org/intermediate-tutorials/billboards-particles/billboards/)

gen_player_buffer() uses gen_faces(10, 6). 6 faces = cube, 4 for two quads, each side. The block-breaking indicator would only need 1 face.

[satoshinm]

Merging what I have for now, separated out the block breaking animation into #106

[satoshinm]

Note, 62b4f94 broke variable hardness since hit_test_face() returns a boolean (1 or 0), not the block type id! (todo: stdbool? good idea?). Changed to use _face() since wanted to draw the block break animation on the face, but GH-106 isn't implemented yet, and it breaks variable hardness, therefore reverted in #108 d502b94 as I found while implementing the block info text, it was always returning 1 not w.