Do less copies of Observables in Attributes + plot pipeline

[SimonDanisch]

While debugging how to update the array for xmax in #2446, i noticed we copy the Observable at least 3 times, making xmax.val = [...] not work, since the copied observables will still hold the old value.
So this should also cut down on observables, which in theory should make updates and creation a bit faster.

[MakieBot]

Compile Times benchmark

Note, that these numbers may fluctuate on the CI servers, so take them with a grain of salt. All benchmark results are based on the mean time and negative percent mean faster than the base branch. Note, that GLMakie + WGLMakie run on an emulated GPU, so the runtime benchmark is much slower. Results are from running:

using_time = @ctime using Backend
# Compile time
create_time = @ctime fig = scatter(1:4; color=1:4, colormap=:turbo, markersize=20, visible=true)
display_time = @ctime Makie.colorbuffer(display(fig))
# Runtime
create_time = @benchmark fig = scatter(1:4; color=1:4, colormap=:turbo, markersize=20, visible=true)
display_time = @benchmark Makie.colorbuffer(display(fig))

	using	create	display	create	display
GLMakie	30.34s (30.06, 31.01) 0.34+-	16.51s (16.10, 17.18) 0.35+-	15.52s (15.13, 16.20) 0.37+-	11.31ms (11.14, 11.56) 0.15+-	129.22ms (127.95, 132.99) 1.84+-
master	29.65s (29.46, 29.98) 0.19+-	16.45s (16.11, 17.24) 0.41+-	15.47s (15.32, 15.73) 0.17+-	13.91ms (13.72, 14.14) 0.13+-	131.22ms (130.03, 132.66) 1.00+-
evaluation	+2.27%, 0.69s slower X (2.53d, 0.00p, 0.26std)	+0.38%, 0.06s invariant (0.17d, 0.76p, 0.38std)	+0.31%, 0.05s invariant (0.17d, 0.76p, 0.27std)	-22.98%, -2.6ms faster✅ (-18.63d, 0.00p, 0.14std)	-1.55%, -2.0ms faster ✓ (-1.36d, 0.03p, 1.42std)
CairoMakie	28.29s (27.96, 28.57) 0.22+-	18.00s (17.79, 18.25) 0.16+-	2.57s (2.52, 2.63) 0.04+-	11.57ms (11.26, 11.91) 0.23+-	4.94ms (4.78, 5.06) 0.10+-
master	27.42s (26.96, 27.76) 0.30+-	17.99s (17.68, 18.31) 0.24+-	2.56s (2.51, 2.63) 0.04+-	14.20ms (14.08, 14.35) 0.09+-	4.70ms (4.60, 4.80) 0.07+-
evaluation	+3.10%, 0.88s slower X (3.29d, 0.00p, 0.26std)	+0.06%, 0.01s invariant (0.06d, 0.92p, 0.20std)	+0.18%, 0.0s invariant (0.12d, 0.83p, 0.04std)	-22.72%, -2.63ms faster✅ (-15.17d, 0.00p, 0.16std)	+4.87%, 0.24ms slower X (2.72d, 0.00p, 0.09std)
WGLMakie	34.70s (34.22, 35.06) 0.27+-	27.00s (26.60, 27.29) 0.20+-	45.84s (43.48, 56.97) 4.93+-	15.14ms (14.73, 15.59) 0.31+-	1.93s (1.90, 1.96) 0.03+-
master	34.27s (34.06, 34.47) 0.15+-	26.86s (26.59, 27.22) 0.22+-	47.42s (43.42, 68.20) 9.17+-	18.83ms (18.28, 19.14) 0.31+-	1.72s (1.70, 1.76) 0.02+-
evaluation	+1.24%, 0.43s slower X (1.97d, 0.00p, 0.21std)	+0.53%, 0.14s invariant (0.68d, 0.23p, 0.21std)	-3.46%, -1.58s invariant (-0.22d, 0.70p, 7.05std)	-24.38%, -3.69ms faster✅ (-11.81d, 0.00p, 0.31std)	+10.71%, 0.21s slower❌ (8.74d, 0.00p, 0.02std)