Brush opacity can exceed 1.0 causing weird painting behavior

[ikonnow]

ArmorPaint version: Latest git build

OS / GPU model: Windows

Issue description:
Brush opacity can exceed 1.0 when using brush nodes or high pressure sensitivity, causing weird painting behavior. The final opacity calculation in uniforms.c multiplies brush_opacity (0.0-1.0) by brush_nodes_opacity and pen_pressure * pressure_sensitivity (which can go up to 10.0), but the result is not clamped.

Steps to reproduce:

1. Set brush opacity to 1.0 via slider
2. Use brush nodes with opacity > 1.0, or
3. Set pressure sensitivity to high values (up to 10.0)
4. Paint with pen pressure enabled
5. Observe weird painting behavior due to opacity > 1.0