[CSSProcessor] Replace utf8_codepoint_at with utf8_ord (#202)

Replaces the `utf8_codepoint_at()` call in `CSSProcessor` with
`utf8_ord()`. This removes the last reference to `utf8_codepoint_at()`
from the codebase so the next PR will remove that function, see
#200 for prior context.

cc @dmsnell

Author: adamziel

components/DataLiberation/URL/class-cssprocessor.php

@@ -2,9 +2,9 @@
 
 namespace WordPress\DataLiberation\URL;
 
-use function WordPress\Encoding\utf8_codepoint_at;
 use function WordPress\Encoding\codepoint_to_utf8_bytes;
 use function WordPress\Encoding\compat\_wp_scan_utf8;
+use function WordPress\Encoding\utf8_ord;
 use function WordPress\Encoding\wp_scrub_utf8;
 
 /**
@@ -1506,7 +1506,7 @@ private function consume_ident_start_codepoint( $at ): int {
 		}
 
 		$codepoint_byte_length = $new_at - $at;
-		$codepoint             = utf8_codepoint_at( $this->css, $at );
+		$codepoint             = utf8_ord( substr( $this->css, $at, $codepoint_byte_length ) );
 		if ( null !== $codepoint && $codepoint >= 0x80 ) {
 			return $codepoint_byte_length;
 		}