Editor GPU: Consolidate CSS rule support and logic

[Tyriar]

These should be co-located

vscode/src/vs/editor/browser/gpu/viewGpuContext.ts

Lines 251 to 268 in e7fb282

	/**
	* A list of supported decoration CSS rules that can be used in the GPU renderer.
	*/
	const gpuSupportedDecorationCssRules = [
	'color',
	'font-weight',
	'opacity',
	];
	function supportsCssRule(rule: string, style: CSSStyleDeclaration) {
	if (!gpuSupportedDecorationCssRules.includes(rule)) {
	return false;
	}
	// Check for values that aren't supported
	switch (rule) {
	default: return true;
	}
	}

vscode/src/vs/editor/browser/gpu/renderStrategy/fullFileRenderStrategy.ts

Lines 395 to 409 in e7fb282

	switch (r) {
	case 'color': {
	// TODO: This parsing and error handling should move into canRender so fallback
	// to DOM works
	const parsedColor = Color.Format.CSS.parse(value);
	if (!parsedColor) {
	throw new BugIndicatingError('Invalid color format ' + value);
	}
	decorationStyleSetColor = parsedColor.toNumber32Bit();
	break;
	}
	case 'font-weight': {
	const parsedValue = parseCssFontWeight(value);
	if (parsedValue >= 400) {
	decorationStyleSetBold = true;