added function that procedurally generates heightfield

[demo] / external / gph-math

diff --git a/external/gph-math b/external/gph-math
index 4c82eda..54ac4cb 160000 (submodule)
--- a/external/gph-math
+++ b/external/gph-math
@@ -1 +1 @@
-Subproject commit 4c82eda2dcba00a4998cfd60a13330dc793b16ca
+Subproject commit 54ac4cbc3b08744e570a53e7f1a8ba862ea87b80