Fix normal coloring for odometry scans and keyframes

[noelex]

Summary

Fix normal-based coloring for odometry scans and keyframes in the standard viewer.

Problem

This fixes a regression introduced during the intensity display refactor in #281.

The odometry NORMAL color mode uploads normal-derived colors through
PointCloudBuffer::add_color(), but the scan shader was configured to use
VERTEX_COLORMAP. As a result, it read the colormap attribute used for
intensity rendering instead of the vertex color attribute.

Odometry keyframes also did not upload normal-derived vertex colors, and their
NORMAL mode continued to use rainbow coloring.

Changes

• Use VERTEX_COLOR for odometry scan normal coloring.
• Upload normal-derived vertex colors for odometry keyframes.
• Use VERTEX_COLOR for keyframes when normals are available.
• Preserve the existing Flat/Rainbow fallback when normals are unavailable.

Testing

• git diff --check
• cmake --build build/glim --target standard_viewer -j2

[koide3]

Thank you!