[egl] shader: fix reliance on null terminated strings

Compiled in resources often will not contain a null terminator, as such
we must not use functions that rely on it. This implements a memsearch
function that performs like strstr on a buffer instead of a null
terminated string.

common/include/common/stringutils.h

@@ -37,4 +37,10 @@ bool str_containsValue(const char * list, char delimiter, const char * value);
 // Local implementation of strdup
 char * lg_strdup(const char *s);
 
+// search a non null terminated buffer for a value
+const char * memsearch(
+    const char * haystack, size_t haystackSize,
+    const char * needle  , size_t needleSize  ,
+    const char * offset);
+
 #endif