Black clipping occurs when using blur effects with GPU rendering

Issue:
When using Mercury Playback Engine GPU Acceleration (CUDA), applying blur effects such as Gaussian Blur or Directional Blur causes black clipping in dark areas, resulting in loss of shadow detail. This problem does not occur when using the Mercury Playback Engine Software Only mode.

Steps to reproduce:

1. Place a clip with shadows or grayscale gradients in the timeline

2. Apply a blur effect with GPU rendering enabled

3. Preview or export shows clipped shadows

Request:

• Improve GPU rendering precision so that tonal range matches software rendering results

• Reintroduce the "Mercury Playback Engine Software Only" option removed in v25.2+, to allow users to bypass GPU rendering when necessary for quality reasons.