Add shadertools as an optional dependency

We should be able to use custom shaders in demos.

Pick-to: 6.5
Change-Id: I5984313c4bf9b7b50d890eddde1d9d0d89eb8dc8
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

dependencies.yaml

@@ -8,6 +8,9 @@ dependencies:
   ../qtmultimedia:
     ref: b5446cebeba3a794aadc32e32eeb1105a33a7b7e
     required: false
+  ../qtshadertools:
+    ref: fd5be631780db8c441561ae3ee4a9295572e3ed0
+    required: false
   ../qttools:
     ref: 7ece380fbb8b8a1357e19e7abb6718ae9b186ba4
     required: true