Compression can't be enabled for PDF/A compliant documents

[schlotzz]

Description:
I've tried to enable compression for documents with PDF/A standard, but was not able to do so. The method setCompression() disables compression as soon pdfa_mode is active.

public function setCompression($compress=true) {
	$this->compress = false;
	if (function_exists('gzcompress')) {
		if ($compress) {
			if ( !$this->pdfa_mode) {
				$this->compress = true;
			}
		}
	}
}

Issue:
Compression cannot be enabled for PDF/A. As stated on Wikipedia, only LZW compression is forbidden within PDF/A standard due to former intellectual property constraints. DEFLATE (a.k.a. "/Filter /FlateDecode") as it is used within TCPDF, is permitted as an open and patent-free standard.

Proposed Solution:
Compression should be allowed for all PDF/A-modes.

I already tested some code changes succesfully and could offer a pull request if wanted.