Brush output node does not allow texture for hardness value

**ArmorPaint version:**
ArmorPaint.org - v0.8 (2021-08-13) - 8131ede

**OS/device including version:**
Win10x64

**Steps to reproduce:**
Build the following brush
The brush will not paint regardless which texture is chosen

Reproduce the brush graph shown in the issue using ArmorPaint v0.8 (8131ede) on Win10x64, then trace how the Brush output node handles a texture-based hardness value. Done means selecting a hardness texture no longer prevents the brush from painting, and the reported graph paints correctly.

Written by the indexing model from the issue text.