Opaque Leaves no longer use distinct background colors

Prior to the texture update, opaque leaves had a separate, darker color (#2c2c2c) as the background (except for jungle leaves, which used #4b4b4b and spruce, which used #313131) to remove transparency while retaining the contrast and detail of the texture. However, this got lost when the new textures were added, so now they just use the darkest color from their own palettes, losing all the detail when playing with fast graphics.

Steps to Reproduce:

1. Place leaves somewhere and observe the detail;
2. Go to the video settings;
3. Disable Fast Graphics (Fast Leaves, if available);
4. Observe the opaque textures.

Observed Results:
The opaque textures unnecessarily lose a lot of detail by not differentiating the background from the leaves. The most noticeable loss of detail is with the birch leaves.

Expected Results:
The background is still clearly defined and contrasts with the leaves, allowing for the same level of detail in the texture to be kept.

Screenshots/Videos attached: Yes

Notes: Mangrove leaves don't have separate opaque textures, so the background will be a full black. Cherry leaves already use a separate (albeit brighter) color.