Advanced image compression: remove posterization

This removes the posterization filter from the advanced image compression
settings to substantially reduce the CPU load which could cause image
resizing to fail (see #150).

After digging into the research @nwtn did to come up with these
settings, it turns out that posterization only improves file sizes
for PNGs and can actually lead to slightly larger file sizes for
JPG images.

class-respimg.php

@@ -29,9 +29,9 @@ class Respimg extends Imagick {
 	 *
 	 * This function is basically equivalent to:
 	 *
-	 * $optim == true: `mogrify -path OUTPUT_PATH -filter Triangle -define filter:support=2.0 -thumbnail OUTPUT_WIDTH -unsharp 0.25x0.08+8.3+0.045 -dither None -posterize 136 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB INPUT_PATH`
+	 * $optim == true: `mogrify -path OUTPUT_PATH -filter Triangle -define filter:support=2.0 -thumbnail OUTPUT_WIDTH -unsharp 0.25x0.08+8.3+0.045 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB INPUT_PATH`
 	 *
-	 * $optim == false: `mogrify -path OUTPUT_PATH -filter Triangle -define filter:support=2.0 -thumbnail OUTPUT_WIDTH -unsharp 0.25x0.25+8+0.065 -dither None -posterize 136 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB -strip INPUT_PATH`
+	 * $optim == false: `mogrify -path OUTPUT_PATH -filter Triangle -define filter:support=2.0 -thumbnail OUTPUT_WIDTH -unsharp 0.25x0.25+8+0.065 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB -strip INPUT_PATH`
 	 *
 	 * @access	public
 	 *
@@ -50,7 +50,7 @@ public function smartResize( $columns, $rows, $optim = false ) {
 		} else {
 			$this->unsharpMaskImage( 0.25, 0.25, 8, 0.065 );
 		}
-		$this->posterizeImage( 136, false );
+
 		$this->setImageCompressionQuality( 82 );
 		$this->setOption( 'jpeg:fancy-upsampling', 'off' );
 		$this->setOption( 'png:compression-filter', '5' );