Previously we assumed that we generally do not need to unbind textures
from shader samplers: if a sampler had no Taisei-level texture binding,
we would simply not update its OpenGL-level binding. This assumption is
no longer valid after introduction of cubemap textures. Shader samplers
always have an OpenGL-level binding to a texturing unit (0 by default).
If the sampler type is not compatible with the type of texture bound to
its texturing unit (e.g. 2D texture with a cube sampler), the draw call
will raise an error, even if the shader does not actually use the
invalid sampler.
For that reason, we now make sure that all samplers without a
Taisei-level texture binding are assigned a texturing unit with no
OpenGL-level texture binding. The texunit juggling logic had to be fixed
quite a bit to correctly handle unbinding textures.
Additionally, some assertions have been added to prevent assigning
incompatible textures to samplers via the renderer API, and the number
of minimum required texturing units has been raised to 8 (we sometimes
need more than 4 bound textures at a time).
b45b2c22ce