ピクセルカラーを座標 (face, x, y) で取得します。
If the pixel coordinates are out of bounds (larger than width/height or small than 0),
they will be clamped or repeat based on the texture's wrap mode.
The texture must have the Is Readable flag set in the import settings, otherwise this function will fail. GetPixel is not available on Textures using Crunch texture compression.