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: #119 (comment)). 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: image-rs/image#1711 (comment)

[anhtumai]

@fschutt btw, do you plan to rewrite this library to not use lopdf? To me, lopdf lacks good documentation, which can be an obstacle for people to contribute.

[fschutt]

No, I don't think I'll switch from lopdf as it's still a pretty good crate.

The reason I wanted my own ColorType is because PDF doesn't support all color types that the image crate supports, the image crate also changes a lot and I don't want to break the API.