Stability of layer data attributes

[teunbrand]

This PR aims to fix #3175.

It avoids dropping attributes in some places and actively restores them in other places.
Attributes should now be stable from the moment aesthetics are computed up to the Geom$draw_*() methods.

[teunbrand]

I cannot figure out why coverage is failing and R CMD check passes.

[yutannihilation]

I too cannot figure out, but I confirmed this error is reproducible on my local. As it looks too cryptic, maybe we can just ignore the failing tests for now? cf. https://covr.r-lib.org/#exclusions

══ Failed tests ════════════════════════════════════════════════════════════════
── Failure ('test-layer.R:177:3'): attributes on layer data are preserved ──────
attr(ld, "test") (`actual`) not equal to "preserve me" (`expected`).

`actual` is NULL
`expected` is a character vector ('preserve me')
── Failure ('test-scales.R:71:3'): identity scale preserves input values ───────
`d1` (`actual`) not equal to `d2` (`expected`).

     names(actual) | names(expected)     
 [1] "shape"       - "size"          [1] 
 [2] "size"        - "alpha"         [2] 
 [3] "alpha"       - "x"             [3] 
 [4] "x"           - "y"             [4] 
 [5] "y"           - "colour"        [5] 
 [6] "colour"      - "fill"          [6] 
 [7] "fill"        - "shape"         [7] 
 [8] "PANEL"       | "PANEL"         [8] 
 [9] "group"       | "group"         [9] 
[10] "stroke"      | "stroke"        [10]

[ FAIL 2 | WARN 1 | SKIP 258 | PASS 1593 ]