Add alpha channel support for latest git clients/servers, don't use inventory cubes

This adds a nice alpha channel to all glass textures, and appropriate single lines of code to enable them.  Now it really looks like stained glass.

The alpha effect works only with the very latest git of minetest, and only if you're using shaders.

Bear in mind the effect is glitchy.  With alpha-capable clients (anything from about 8 hours ago or newer), it exhibits layering errors/glitches - exactly what the glitch is depends on whether you have shaders turned on, and the angle you're looking at the glass from.  With shaders on, you may see direction-dependent layering errors between the glass and other translucent objects, but it's quite playable and nice to look at anyway.  Without shaders, the clouds will sometimes appear in front of the glass, much like the problem we sometimes have with water fountains.

Without an alpha-capable client (e.g. 0.4.6-stable) it should gracefully fallback and look about the same as it did before.

Also, regarding the inventory cubes, this function is deprecated and should never be used in practice, as it prevents the engine from rendering dropped items as actual cubes (ironically).  Removed those calls from the code.

![screenshot_1346055471](https://f.cloud.github.com/assets/1733537/435625/daace1da-aff0-11e2-960c-c6bf13120512.png)