This was done because the backdrop as implemented was the only texture in the entire game that had to be deleted and recreated each frame.
However, with Vulkan this would have necessitated quite a bit of synchronization with the render pipeline which wasn't really feasible just for this one single texture.
Now the texture manager can assume that once a texture was created it will be immutable and never has to change.
This removes 3 uniforms, consisting of 9 floats. Those were merged into other values that never get used at the same time.
It also moves the costly setup of the fixed colormap out of the render state into the 2D processing code.
Since 3D forces use of render buffers now, it is no longer necessary to draw the entire scene with the colormap active, meaning it can be handled more efficiently.
It may use the same calculations as the hardware renderer but must use the 2D drawer for display.
It should be investigated if the hardware renderer can do this as well.
This also replaces DTA_ColormapStyle with proper implementations of its components. As implemented it was a very awkward mixture of various effects that already existed in a separate form. As a result of its implementation it required additional but completely redundant shader support which could be removed now. As a side effect of this change a new DTA_Desaturate option was added.
