This change adds special colors to color maps that can optionally be
used for scalar values outside the range of the color map. It touches
a number of classes in VTK.

vtkLookupTable

- Added AboveRangeColor for values above the lookup table range.

- Added UseAboveRangeColor option to control whether the
  AboveRangeColor or the color that maps to the highest value is used
  for values above the range.

- Added BelowRangeColor for values below the lookup table range.

- Added UseBelowRangeColor option to control whether the
  BelowRangeColor or the color that maps to the lowest value is used
  for values below the range.

- Made GetColorAsUnsignedChars() a static member so that other classes
  can use it.

TestLookupTable

- Modified test to include testing of {Above|Below}RangeColors
  functionality.

TestOutOfRangeColorTransferFunction.py

- Test of out-of-range color functionality in
  vtkColorTransferFunction.

TestOutOfRangeDiscretizableColorTransferFunction.py

- Test of out-of-range color functionality in
  vtkDiscretizableColorTransferFunction.

vtkScalarsToColors

- Made RGBABytes member protected so subclasses can use it.

vtkColorTransferFunction

- Added {Above|Below}RangeColor and Use{Above|Below}RangeColor member
  variables that function the same as the members with the same name
  in vtkLookupTable. These colors are specified as RGB colors instead
  of RGBA colors, as they are in vtkLookupTable. For better or worse,
  this is for consistency with how NanColor is specified in each of
  these classes.

- Above and BelowRangeColor are used only if the Clamping option is
  enabled.

vtkDiscretizableColorTransferFunction

- Forwards {Above|Below}RangeColor and Use{Above|Below}RangeColor to
  the internal vtkLookupTable in the Build() method.

- Fixed a bug that caused the internal vtkLookupTable to allocate
  3 times the amount of colors needed to hold the table.

vtkScalarsToColorsPainter

- Added two colors for {Above|Below}RangeColors in the lookup table
  texture map. The color map class fills in these colors appropriately
  based on the {Above|Below}RangeColors settings.

- Fixed the texture coordinate calculation to accommodate out-of-range
  colors.

Performance-wise, this change increases the time to map an array with
10 million scalars to colors by 5.5% using
vtkLookupTable::MapScalarsThroughTable2().

Change-Id: Ia3535fae175e991c0d1a4cf70752845193b4166d