Perf/reduce mem allocations

[cmeissl]

attempt on reducing mem allocations in hot-code paths and some performance improvements

not sure how to best handle the Space::refresh/Output::enter/Output::leave stuff. this is called way to often
imo and we need to get the alive checks out of enter/leave.

[Drakulix]

A couple of small nitpicks and questions, but otherwise this looks like a really nice improvement so far.

[Drakulix]

This will cause new Vec's to be created, no? Why are we not directly using self.damage here, we just cleared it anyway?

[cmeissl]

It is returned here:

smithay/src/backend/renderer/damage/mod.rs

Line 681 in 13bc424

std::mem::swap(&mut self.damage, &mut damage);

and an empty Vec does not allocate. Anyway, this is also just some leftover from searching for a error and we can just use self.damage, yeah.

[Drakulix]

Right and empty Vec does nothing, but it is still confusing to read. If there is no reason to do it this way, I would prefer to just use the value directly in all these cases.

[cmeissl]

mostly done, we need to take element_damage as we move it around a bit

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 86 lines in your changes are missing coverage. Please review.

Project coverage is 20.50%. Comparing base (47a8d5b) to head (f4d5874).

Files	Patch %	Lines
src/backend/renderer/utils/mod.rs	55.88%	30 Missing ⚠️
src/backend/renderer/damage/mod.rs	69.86%	22 Missing ⚠️
src/wayland/output/mod.rs	50.00%	11 Missing ⚠️
src/backend/renderer/element/memory.rs	0.00%	6 Missing ⚠️
src/backend/renderer/element/texture.rs	0.00%	5 Missing ⚠️
src/backend/renderer/element/utils/elements.rs	0.00%	5 Missing ⚠️
src/backend/renderer/element/mod.rs	33.33%	4 Missing ⚠️
src/backend/renderer/utils/wayland.rs	84.61%	2 Missing ⚠️
src/backend/renderer/damage/shaper.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1346      +/-   ##
==========================================
- Coverage   20.61%   20.50%   -0.11%     
==========================================
  Files         161      161              
  Lines       26145    25987     -158     
==========================================
- Hits         5390     5329      -61     
+ Misses      20755    20658      -97     

Flag	Coverage Δ
wlcs-buffer	17.91% <59.53%> (-0.13%)	⬇️
wlcs-core	17.60% <54.41%> (-0.11%)	⬇️
wlcs-output	7.32% <14.41%> (-0.16%)	⬇️
wlcs-pointer-input	19.47% <53.48%> (-0.11%)	⬇️

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