First step toward rendering multiple objects (using different SDF shaders)

(I want to draw multiple things in a row without having them each
clear the screen.)

Remove csubst now that it's part of base C lib

multiple of a shape

Also fix linebreaks in csubst

(next, update images, which is harder)

Now we can standardize on struct representation of C structs in Tcl
instead of dict representation... hopefully simpler code too in the end.

WIP: Start on images/sampler2D.
Improve char array support in C. Actually put uboFields in Pipeline
struct so they don't shimmer out (because we now run the Pipeline data
back into C to set up the image right before draw time).