Support rotating the layouts

[fedarko]

Brought up by @kwcantrell in kwcantrell#49 (comment) and kwcantrell#49 (comment).

This would be really nice for the rectangular layout (since it'd let users draw the tree going from top to bottom, which is a common way of representing them) and for the circular layout (if the user wants the "start point" of the circle to be at an angle other than 0 radians).

This might break the "maximum displacement" computations for rectangular barplots a bit, but fixing it probably won't be too bad. (It might be easier to just support increments of 90 degrees for the rectangular layouts; not sure right now.)

[kwcantrell]

Actually, we would not need to modify anything. The rotation will be done in the shades so as far as empress is concerned, the tree will not actually change.

To rotate, all we have to do is multiply the world matrix by a rotation matrix. (The world matrix is currently responsible to zooming/moving the tree)

[fedarko]

Oh wow. That makes life even easier! So this would automatically rotate the barplots buffer, as well?

[kwcantrell]

Yupe, it will rotate everything! On top of that, it is a "free operation" in the sense that no extra computation resources are needed. WebGl will be doing the same amount of work whether or not the tree is rotated. The "cost" of rotating the tree is simply a single 4x4 matrix multiplication.