Added descriptor_heap sample

samples/descriptor_heap/shaders/heap_triangle.frag

@@ -0,0 +1,9 @@
+#version 450
+
+#pragma shader_stage(fragment)
+
+layout(location = 0) out vec4 outColor;
+
+void main() {
+    outColor = vec4(0.f, 1.f, 0.f, 1.f);
+}