Disable depth mask when rendering custom blocks

[deathcap]

From mikolalysenko in #2:

When rendering the world you need to do 2 passes:

• First draw all opaque/transparent voxels without using any blending and with the depthMask enabled.
• Then disable depthMask (but keep the depthTest on!), turn on blending and draw all transparent faces.

or http://stackoverflow.com/questions/3388294/opengl-question-about-the-usage-of-gldepthmask, similar process:

1. Turn on the depth mask glDepthMask( GL_TRUE )
2. Draw all opaque objects, in any order
3. Turn off the depth mask glDepthMask( GL_FALSE )
4. Turn on a BLEND_MODE
5. Draw translucent objects sorted from furthest away to nearest

Currently, voxel-shader draws all opaque/transparent objects first, then translucent objects with blending enabled. However, the meshes are not sorted (voxel/voxel-mesher#9) and the depth mask is not enabled or disabled (this issue, GH-8).

https://www.opengl.org/sdk/docs/man/html/glDepthMask.xhtml

[deathcap]