tsc considers everything in the else block to be of type 'never',
since it thinks that cosh is permanently defined for Math. Casting
back to Math in the else block fixes the error.
Remove 'extent' because the parent Source class does not contain
'extent' on its options or use options.extent in the constructor.
Also removed 'extent' from 'Options' for 'TileSource'.
Fixed constructor docs specifying optional options without a
default value. Fixed malformed @link tag in typedef description.
canvasFunction type was previously declared with the 'this' type
as ImageCanvas, but the 'this' ref was not passed on the scope when
calling the function.
The parent class does not accept an extent on its options. Also
check that the image is an img or video tag before setting the
source as Canvas does not have the src property.
I'm not entirely sure that I like this pattern, but there does not
appear to be a better option since TypeScript would recommend:
function(this: SomeClass, ...)
tsdoc does not appear to support an associated annotation for the
function a la @this {SomeClass}.
William Wall