replace color_space field in `ImageXObject` with `color_type` field

[anhtumai]

ColorSpace is just a simplified version of ColorType and is not used much in this library codebase.

Replacing it with the color_type field in ImageXObject will give library users more information, in case they want to do some transformation with the field image_data.

For example, some people reported that adding an image with the Alpha channel will result in a blank page (#84, #119). This can be resolved by removing the alpha channel from ImageXObject::image_data (check this comment: https://github.com/fschutt/printpdf/issues/119#issuecomment-1120434233). However, for different color_type, image_data will have different formats. For ColorType::Rgba8, image_data will have format: [Red1, Green1, Blue1, Alpha1, ..., RedN, Green2, Blue2; AlphaN]. But for ColorType::Rgba16, the format will be [Red1a, Red1b, Green1a, Green1b, Blue1a, Blue1b, Alpha1a, Alpha1b, ...]. Source: https://github.com/image-rs/image/issues/1711#issuecomment-1120450753