perf: store conic form in projection cache to avoid per-tile conversion

[mvaligursky]

Store conic-form values (cx, cy, cz) directly in the projection cache instead of the Gaussian evaluation coefficients (coeffX, coeffY, coeffXY). This eliminates the per-tile conversion previously done inside tileIntersectsEllipse, saving a few multiplies per splat per tile in both TileCount and Scatter passes.

Changes:

• computeSplatTileEval and tileIntersectsEllipse now accept conic form (cx, cy, cz) directly instead of coeffX/coeffY/coeffXY, removing the internal conversion
• TileCount pass computes and stores cx/cy/cz in the projection cache
• Scatter pass reads cx/cy/cz and passes them directly to tile intersection
• Rasterize pass reads cx/cy/cz and converts back to evaluation coefficients (coeffX = -cx/2, coeffY = -cz/2, coeffXY = -cy) for per-pixel Gaussian evaluation