Explicitly initialize values to 0 in vtkImageCanvasSource2D when creating
the alpha component for backgroundAlpha

vtkAlgorithm now supports ability to specify values for shifting and
scaling progress. This enables internal vtkAlgorithm instances to report
progress for part of the work without too much work. Also allows
vtkCompositeDataPipeline to report progress was each iteration over the
composite dataset as a part of the whole rather than having the progress
be from [0, 1.0] for each block in the composite dataset.

The flag CompoundAlpha affects the blending computation in the compound
blending mode of vtkImageBlend. When the flag is off, vtkImageBlend
behaves as it used to and the output alpha values are the sum of the
product of individual opacity and alpha values in the inputs i.e.
output alpha A = (O1A1 + O2A2)

When the flag is on, the blend computation is also applied to the alpha
component of the inputs i.e.
output alpha A = (O1A1A1 + O2A2A2) / (O1A1 + O2A2)

In 2 cases, made trivial use of NewInstance, so that it’s not unused.

Found via `codespell`

The code for ChangeExtent would only call SetExtent() if the Y or
Z values had changed.  If the only change was to X, then Extent
would not be modified.

- Changed some to be const also.
- Replaced others with call to ClampValue method.
- Changed some from int to bool.
- In a couple of cases the variable was unused, removed it.

Found via `codespell`

Found via `codespell`

vtkImageBlend was not blending images correctly in compound mode
for 2-channel (LA) and 1-channel images.
Test case has been added to test the compound mode, which
was previously missing.

Found via `codespell`

Change array syntax from pointer to dynamic array
Add size hints for dynamic arrays

This filter is most often to apply a Window/Level operation to
grayscale input data, without a lookup table, so I have put that
first in the description.  The less-common use of modulating the
color produced by a lookup table has been moved to the end.