Perfmatters 2.3.6

* Remove Used CSS filter adjustment to fix an issue where certain WordPress post functions wouldn't be available when trying to selectively disable the feature.
* Rolled back minor plugin UI JavaScript addition, as it was interfering with entering data on multiple lines in certain input fields.

* Added new perfmatters_used_css filter.
* Added new perfmatters_allow_buffer filter.
* Added a notice in the Script Manager when Testing Mode is enabled.
* Improved reliability of CSS Background Image function when child elements with additional background images are present.
* Script Manager style compatibility fixes.
* Fixed an issue where some post specific meta options were not being respected when determining if a feature should run.
* Fixed an issue where pressing enter on the main plugin settings page would trigger a specific form action instead of save settings.
* Changed CSS class initialization hook to be in the correct order with other output buffer functions.
* Made an adjustment to how we generate the local used stylesheet URL for better compatibility.
* Fixed an issue where loading attribute was still getting applied to images that were excluded from lazy loading.
* Fixed an issue where images inside an excluded picture element were not also getting excluded.
* Fixed an issue in the Script Manager where archives were not being grouped together with their respective post type.
* Additions to plugin UI JavaScript to allow for disabled sections to be hidden even when nested controllers are present.
* Moved background process library to composer autoloader.
* Removed BETA tag from Remove Unused CSS option.

* Added new option to lazy load CSS Background Images.
* Added new option for Dual Tracking when using gtag.js in local analytics.
* Added new perfmatters_rest_api_exceptions filter.
* Fixed an issue where individually delayed local scripts would not get correctly rewritten to load from the CDN.
* Fixed an issue where lazy loading would run into an error if no px or % was specified with the threshold value.
* Fixed an issue with buffer validation that was conflicting with certain caching setups.
* Fixed an issue where existing font preconnect and prefetch tags were not being being detected properly when using Local Fonts.
* Fixed an error related to cookie constants when running MU Mode in certain environments.
* Fixed multiple AMP validation errors and added additional checks to prevent certain functions from running on AMP URLs.
* Minor adjustment to CDN rewrite regex pattern to work with encoded quotation characters.
* Changed toggle CSS selectors to be more specific to prevent conflicts.
* Moved plugin settings header output to in_admin_header action hook for compatibility.
* Moved JS optimization functions to new class structure to be more inline with current codebase.
* Improvements to critical image preloading allowed for a move to a singular output buffer.