Create alpha layer with boundaries matching content

[pflammertsma]

Related to #4.

Currently, when an alpha layer is created for a group with opacity value < 1, the boundaries are calculated to match the dimension of the entire canvas. This is especially problematic when scaling is applied to the matrix, as the off-screen bitmap can become quite sizable, with poor performance and ultimately may lead to OOMEs.

The most obvious way to combat this is by computing the boundaries of the paths inside the group, and creating the layer to match the left, top, right and bottom coordinates of the boundary.