[client] egl: use new util_hasGLExt helper to check extensions

We previously used strstr, which can be prone to false positives when
the name of one extension is a substring of another extension.

This commit creates the helper function util_hasGLExt, which asserts
that the substring found in extension list is bounded by either spaces
or the beginning/end of the string.

client/src/util.c

@@ -21,9 +21,11 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #include "main.h"
 
 #include "common/debug.h"
+#include "common/stringutils.h"
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <assert.h>
 #include <math.h>
 
@@ -207,3 +209,8 @@ void util_rotatePoint(struct DoublePoint *point)
       break;
   }
 }
+
+bool util_hasGLExt(const char * exts, const char * ext)
+{
+  return str_containsValue(exts, ' ', ext);
+}