Make elementCallback idempotent if using a custom viewport width

[martinbooth]

I hadn't realised this got called multiple times with the same node and it isn't currently idempotent so each time a component re-renders it re-wraps getBoundingClientRect, increasing the amount the returned rect is scaled by each time.

Issue is only present if viewportScale !== 1

Fix captures the original getBoundingClientRect in a WeakMap so subsequent calls will re-use the original function when wrapping

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

Results	Base	Patch	Ratio
react-strict-dom/dist/dom/index.js
· compressed	2,514	2,514	1.00
· minified	8,691	8,691	1.00
react-strict-dom/dist/dom/runtime.js
· compressed	855	855	1.00
· minified	2,435	2,435	1.00
react-strict-dom/dist/native/index.js
· compressed	16,675	16,664	1.00	-
· minified	63,767	63,915	1.00	+

[github-actions]

workflow: benchmarks/perf (native)

Comparison of performance test results, measured in operations per second. Larger is better.

Results	Base	Patch	Ratio
css.create
· small	1,123,044	1,140,184	1.02	+
· small with units	488,131	498,086	1.02	+
· small with variables	634,577	672,098	1.06	!!
· several small	345,805	354,334	1.02	+
· large	201,491	202,566	1.01	+
· large with polyfills	146,996	148,399	1.01	+
· complex	102,713	103,567	1.01	+
· unsupported	213,208	214,298	1.01	+
css.createTheme
· simple theme	226,031	226,108	1.00	+
· polyfill theme	210,707	211,144	1.00	+
css.props
· small	230,355	230,309	1.00	-
· small with units	189,069	189,990	1.00	+
· small with variables	102,500	103,288	1.01	+
· small with variables of units	77,257	77,491	1.00	+
· large	96,641	96,910	1.00	+
· large with polyfills	37,325	37,299	1.00	-
· complex	23,694	23,501	0.99	-
· unsupported	143,137	142,787	1.00	-
· simple merge	161,082	161,871	1.00	+
· wide merge	17,875	17,796	1.00	-
· deep merge	17,504	17,412	0.99	-
· themed merge	32,199	32,280	1.00	+