Fixed some assertions we were hitting in debug builds. Added some FIXMEs.
Make sure filter property doesn't accidentally accept transform functions.
Refactor ParseSingleFilter() to have consistent return locations.
Add UngetToken to a return false case in CSS parser.
Pass style context and pres context to SetCoord so we can resolve percent lengths (and avoid some assertions).
Fix bug where we were getting a token, and then ParseVariant was  also getting a token when parsing urls, causing it to fail.
Clear the filter TArray when we see an 'initial' CSS value in order to reset it and have it compute to 'none'.
Add shadow resolving for drop-shadow
Add computedStyle code for new CSS filter functions (drop-shadow still missing). Disabled by default for now.
Make filter functions with number|percentage work again. Do not support calc() on angle yet.
Allow calc on number values for filter functions. (preparation; calc does not support unitless values yet.)
Clean up code