Override default Encoders

[MistaGreen]

Description

Override default Encoders for existing data classes. I've added prepend methods to Encoder and ResourceEncoder classes to achieve this.

Motivation and Context

I want to encrypt images in Glide disk cache. To do that I need to be able to replace Encoders for existing data types like Bitmap, InputStream, Drawable, etc.

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[MistaGreen]

I signed it

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[googlebot]

CLAs look good, thanks!

[sjudd]

Returning false will cause the load to fail. There isn't currently any logic that tries Encoders in order, it just assumes there's one per data type.

[sjudd]

glide/library/src/main/java/com/bumptech/glide/load/engine/SourceGenerator.java

Line 74 in 35c28c2

Encoder<Object> encoder = helper.getSourceEncoder(dataToCache);

[sjudd]

Same here:

glide/library/src/main/java/com/bumptech/glide/load/engine/DecodeJob.java

Line 627 in 35c28c2

diskCacheProvider.getDiskCache().put(key,

[sjudd]

Other than the two minor comments I made, looks good. Thanks!

[MistaGreen]

I fixed comments to describe current behaviour. Thanks!

[sjudd]

LGTM, thanks!