mkmaterial: Blender-related improvements

[snacchus]

This PR adds the following new properties to mkmaterial:

• blender.mode.raw: Similar to combiner.*.raw, in that it allows to specify the exact blender formula (1-cycle or 2-cycle)
• Several properties that control the BLEND_COLOR and FOG_COLOR RDP registers:
  • blender.reg.blend.rgb: Corresponds to the RGB portion of the BLEND_COLOR register. Note that the alpha portion is not settable through a similar property, because this is already covered by rm.alpha_compare.
  • blender.reg.fog.rgb: Corresponds to the RGB portion of the FOG_COLOR register.
  • blender.reg.fog.alpha: Corresponds to the alpha portion of the FOG_COLOR register. This property must not be set if blender.mode is set to multiply_const, because the constant value also takes up the fog alpha.

To make it possible to set RGB and alpha portions of RDP registers independently, some new API has been added at the rdpq-level:

• rdpq_set_blend_color_rgb
• rdpq_set_blend_color_alpha
• rdpq_set_fog_color_rgb
• rdpq_set_fog_color_alpha

These functions are implemented by tracking the current register contents in DMEM (of the rdpq overlay).

[Dragorn421]

Looks good to me! Thanks!

Regarding the weirdness around blender.mode.const and blender.reg.fog.alpha both referring to the same value in fine (which it seems you handled correctly by erroring if both are set), one thing to note is that blender.mode.const currently has no effect (doesn't get exported into the material)
It may be a good opportunity to either get rid of blender.mode.const or actually implement it too

[snacchus]

@Dragorn421 blender.mode.const did indeed have no effect thus far, but I actually fixed that too in this PR. See Blender::get_fog_alpha