Allow decoders to load metadata only

[vpenades]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

In some scenarios, it could be interesting to load the MetaData information of an image, without loading the bitmap data.

The advantage is to be able to load the metadata of thousands of images much faster, and with less memory consumption.

The implementation would be done in the image decoders, with a LoadMetadata method or something like that.

As a bonus, it could be great to be able to modify the metadata of an image file, without modifying the actual compressed image.

[antonfirsov]

@vpenades we have Image.Identify() for this since the merging of #292. Do you lack some other functionality here, or just missed it? :)

[antonfirsov]

@vpenades see this spontaneous design discussion to see API choices we made.

[vpenades]

@antonfirsov ahhh, I didn't know about that one!

[JimBobSquarePants]

As a bonus, it could be great to be able to modify the metadata of an image file, without modifying the actual compressed image.

That would be a very nice bonus. Probably for V2, Lets close this issue and open another for that specific functionality.