Fix clang -Wc++11-narrowing warnings.

Clang compiler gives warning to implicit conversions of
literal structure fields, although the compiler generated
implicit conversions are what we want in these cases.

Change-Id: I8f04e7f2fa1db0ed1d1937135d9091532eec3d5b
diff --git a/opengl/tests/fillrate/fillrate.cpp b/opengl/tests/fillrate/fillrate.cpp
index 1d9b026..2db63d7 100644
--- a/opengl/tests/fillrate/fillrate.cpp
+++ b/opengl/tests/fillrate/fillrate.cpp
@@ -91,11 +91,13 @@
          }
      }
 
+     const GLfloat fh = h;
+     const GLfloat fw = w;
      const GLfloat vertices[4][2] = {
-             { 0,  0 },
-             { 0,  h },
-             { w,  h },
-             { w,  0 }
+             { 0,   0  },
+             { 0,   fh },
+             { fw,  fh },
+             { fw,  0  }
      };
 
      const GLfloat texCoords[4][2] = {