Support modern CSS

[carlosame]

Although EchoSVG supports level 4 selectors, the rest of CSS is only supported at CSS 2. Current SVG images tend to use modern CSS, for example:

• Custom properties (see Render MermaidJS diagrams #35).
• Level 4 values and units (including calc() and other functions).
• Media queries.

This issue is opened to discuss the transition of this project to a more modern CSS. The intention is to migrate the current CSS Object Model to something based on CSS4J's Object Model.

[carlosame]

PR #60 was just merged, and provides a powerful workaround that allows transcoding documents that use modern CSS.

[carlosame]

Summarizing the CSS progress since this issue was opened:

1. The CSSTranscodingHelper was added, and it allows to bypass the native CSS processing and use css4j's Object Model instead.

2. Support for CSS Color Level 4.

3. Improved spec compliance in error handling.

Now that advanced CSS processing is possible thanks to CSSTranscodingHelper, and CSS support in general was improved, instead of keeping this very generic issue open forever I'm closing it. Narrower individual issues shall be opened instead.