Pattern/Shading transformation matrix issues

[brendandahl]

Currently, when we paint patterns/shadings we apply a base transform then apply the pattern's matrix. This isn't technically correct, instead we should leave the current transform applied and instead use pattern.setTransform. Something like:

matrix =  ctx.mozCurrentTransformInverse;
if (!svgElement) {
  svgElement = document.createElementNS("http://www.w3.org/2000/svg", "svg");
}
let mtx = svgElement.createSVGMatrix();
mtx.a = matrix[0];
mtx.b = matrix[1];
mtx.c = matrix[2];
mtx.d = matrix[3];
mtx.e = matrix[4];
mtx.f = matrix[5];
mtx = mtx.translate(clipped.offsetX, clipped.offsetY);
pattern.setTransform(mtx); 

[brendandahl]

A good PDF for testing this:
#6769 (comment)
And also another PDF that is the same as above but with a pattern matrix of [2, 0, 0, 2, 0, 0] that illuminates some other issues as well:
issue6769_no_matrix.pdf