Brush opacity can exceed 1.0 causing weird painting behavior

**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

Start in uniforms.c and inspect the final brush opacity calculation described in the issue, including brush_opacity, brush_nodes_opacity, and pen pressure. Reproduce with high node opacity or pressure sensitivity, then verify that painting remains well behaved and the resulting opacity does not exceed 1.0.

Written by the indexing model from the issue text.