Allow materials to read from a depth texture

[MoritzBrueckner]

Requries armory3d/iron#160.

This PR implements what was discussed in #1817. Materials now can specify that they want to read from the depth buffer. In this case, meshes with those materials are drawn last and before they are drawn, the depth buffer is copied to a dedicated depth texture that can then be sampled from a shader uniform called depthtex.

It works on deferred and forward (Depth prepass on/off), with shadows and transparency, SSR and mesh batching. The only limitation I know is that on forward RP this doesn't work when the compositor is turned off because then the rp_render_to_texture define is undefined and lbuffer0 does not exist. I'm not sure whether this define should only be set for the compositor (as it also creates some other render targets for example) or whether it can be enabled for this feature as well. In general it's difficult to see what rendertargets and defines are used for what purpose, so please let me know if I should change something. Btw, what does the l in lbuffer0 stand for?

Tested on:

• Krom on Windows
• html5
• Hl/C on both OpenGL and D3D11 on Windows

The entire depth texture feature can be toggled on or off in the renderpath settings:

My next goal is to implement the possibility to allow defining samplers/texture units in custom materials via the UI (because they are defined in the mat data .json and not in the shader .json that the user can provide; also Khamake needs to know about the textures). Then it would be possible for example to create an underwater caustics volume together with the depth read feature.

[luboslenco]

Btw, what does the l in lbuffer0 stand for?

I believe it originally comes from the http://advances.realtimerendering.com/s2016/Siggraph2016_idTech6.pdf, standing for lightbuffer.