Avoid breaking existing consumers of props by providing default viewport width

[martinbooth]

No description provided.

[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	16,607	16,663	1.00	+
· minified	63,761	63,763	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	1,130,474	1,150,698	1.02	+
· small with units	495,680	497,688	1.00	+
· small with variables	669,009	660,051	0.99	-
· several small	354,887	356,069	1.00	+
· large	202,235	203,457	1.01	+
· large with polyfills	147,677	149,785	1.01	+
· complex	102,908	103,745	1.01	+
· unsupported	215,092	213,815	0.99	-
css.createTheme
· simple theme	226,609	227,046	1.00	+
· polyfill theme	212,889	213,146	1.00	+
css.props
· small	231,068	229,443	0.99	-
· small with units	190,279	190,368	1.00	+
· small with variables	102,126	102,863	1.01	+
· small with variables of units	77,476	77,673	1.00	+
· large	95,710	96,549	1.01	+
· large with polyfills	37,476	37,299	1.00	-
· complex	23,803	23,381	0.98	-
· unsupported	142,198	143,404	1.01	+
· simple merge	161,802	162,061	1.00	+
· wide merge	17,913	17,761	0.99	-
· deep merge	17,578	17,520	1.00	-
· themed merge	31,886	32,075	1.01	+

[necolas]

Sorry didn't realise you put this PR up. I wanted to make sure the unit tests were updated to capture this potential regression though #349