Evas filters: Implement Lua classes for colors & buffer

Reuse previous code for buffer. Keeps API stability.

The new class "color" is here for a more convenient color
representation. This way, colors can be represented in more
natural ways like: {r,g,b[,a]}, 0xaarrggbb, "red", "#rrggbb"

Class color is implemented in pure Lua, and adds a .lua file
to Evas' share folder.