Allow filter search in epson-escpr-wrapper

Instead of calling epson-escpr from wrapper in hard coded CUPS_FILTER_PATH,
this patch allows wrapper to search filter in non-standard location by calling
cups API cupsSearchFilterLatest:

https://chromium-review.googlesource.com/c/518931/

--- a/src/wrapper.c
+++ b/src/wrapper.c
@@ -33,6 +33,8 @@
 #include <ctype.h>
 /* added 22-04-2004 */
 #include <signal.h>
+/* added 07-09-2017 */
+#include <linux/limits.h>
 
 #include "libprtX.h"
 
@@ -282,10 +284,42 @@ main (int argc, char *argv[])
 
 		if (pfp == NULL)
 		{
-			char tmpbuf[256];
+			char tmpbuf[256 + PATH_MAX];
 
+			char cups_filter_path[PATH_MAX]; // filter full path.
+			// Fill in default filter path (2 additional characters: '/' and '\0').
+			int total_size = snprintf(cups_filter_path, sizeof(cups_filter_path),
+					          "%s/%s", CUPS_FILTER_PATH, CUPS_FILTER_NAME);
+			if (total_size < 0 || total_size >= sizeof(cups_filter_path))
+				return 1;
+			if (access(cups_filter_path, F_OK) != 0)
+			{
+				// Filter does not exist in default location.
+				int status = _cupsSearchFilterLatest(
+					"epson-escpr",
+					cups_filter_path,
+					sizeof(cups_filter_path));
+				if (status == 1)
+				{
+					// found a filter.
+					char *end_of_basedir = strrchr(cups_filter_path, '/');
+					if (!end_of_basedir)
+						return 1;
+					// cut the filename off the filter path.
+					*end_of_basedir = '\0';
+				}
+				else {
+					debug_msg("_cupsSearchFilterLatest failed: %d", status);
+					return 1;
+				}
+			}
+			else {
+				// cut off filename (CUPS_FILTER_NAME).
+				strncpy(cups_filter_path, CUPS_FILTER_PATH,
+					sizeof(cups_filter_path));
+			}
 			sprintf (tmpbuf, "%s/%s \"%s\" %d %d %d %s %s %s %s %s %s %s %s",
-				 CUPS_FILTER_PATH,
+				 cups_filter_path,
 				 CUPS_FILTER_NAME,
 				 fopt.model,
 				 header.cupsWidth,