Add render_mode that allows materials with scissor/hash opacity to cast shadows that ignore their opacity

[Bordraw]

Something that I have run into recently and haven't been able to find a good workaround for: I am writing a shader that fades out walls between the camera and the player using some vertex magic. I am using alpha scissor for this for performance reasons but when the walls fade out, their shadows do too. It would be incredibly helpful if we had flags like 'shadow_draw_always' and 'shadow_draw_opaque' that let us customize this behavior so that we have fine control over how shadows are cast when using discard transparency (i.e. alpha scissor or hash).

[Calinou]

This can be addressed if you can query whether you're currently in the shadow pass: #4443