[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/include/util.h

@@ -32,6 +32,7 @@ void util_cursorToInt(double ex, double ey, int *x, int *y);
 bool util_guestCurToLocal(struct DoublePoint *local);
 void util_localCurToGuest(struct DoublePoint *guest);
 void util_rotatePoint(struct DoublePoint *point);
+bool util_hasGLExt(const char * exts, const char * ext);
 
 static inline double util_clamp(double x, double min, double max)
 {