Alt to #989. See #1065

Intro

This drops our system of per-renderer blend modes, instead using per-object blending. We managed to keep the "weighted" and "dithered" blending modes. The multi-pass "ordered2" and "weighted_plus" are gone. We include examples that demonstrate how these can be mimicked.

The new system:

• allows mixing objects with different blending in the same scene.
• allows more control over blending, by controlling e.g. depth_write and other props.
• is more performant.

But also:

• the user may have to be more aware of opaque vs transparent fragments to avoid artifacts.

Context

You may ask why we have different blend modes in the first place. The short answer is that blending semi-transparent objects is really hard and there is not a single solution that does the trick. And then there are some more special types of blending, like additive.

In the previous system we kind of tried to offer a few blend modes that did their best to "fix" blending for the user. The new system accepts that transparency is non-trivial, relies on the user to think about the problem, and it provides the user with more tools to approach the problem.

Ordered2 blending

The previous default blending was "ordered2", which had the great feature that distinguishing between opaque and transparent fragments happens per-fragment instead of per object. It did this by rendering most objects twice,
discarding transparent fragments in the first pass, and opaque fragments in the second. This solves a range of problems, (but not everything).

The costs are quite high:

• The code is more complex.
• Objects are rendered twice, which is not good for performance.
• The shader had a conditional discard, which prevents the GPU from doing early-z optimizations.

See #1003 for more details.

API changes

• Removed WorldObject.render_mask.
• Removed Renderer.blend_mode, use Material.blending instead.
• Added Material.blending.
• Added Material.transparent, default is None (auto).
• Added Material.depth_write, default is None (auto).
• Added `Material.depth_compare, default is "<".
• Added Material.alpha_test.
• Added Material.alpha_compare, default is "<".
• Removed material.color_is_transparent for Line, Mesh, Points, Text. Use material.color.a < 1 instead.
• Removed material.edge_color_is_transparent for Points. Use material.edge_color.a < 1 instead.
• Removed material.outline_color_is_transparent for Text. Use material.outline_color.a < 1 instead.

Other changes

• The renderer.sort_objects is True by default.
• The renderer's sorting depends on opaque vs transparent objects, and the performance is improved.
• Add support for 'multiply' and 'subtractive' blending.
• More control over 'weighted' blending so it's use extends to other use-cases.
• Background objects have a default render_order of -1e6, so they are usually drawn first.