externalGraphic rendering broken on Android 2.1

[ahocevar]

Looking at http://openlayers.org/dev/examples/vector-features.html (as well as other examples with external graphics, especially the mobile-jq.htm and mobile-sencha.html ones), we can see that both scale and position of externalGraphic styled features are incorrect when viewed with the Android 2.1 default browser.

For the 2.11 release, if we cannot fix this issue, I think it would be nice to make the mobile-jq and mobile-sencha examples use a simple vector symbol instead of an externalGraphic, at least if we detect that the OS is Android 2.1.

[ahocevar]

This is caused by  http://code.google.com/p/android/issues/detail?id=5141

[ahocevar]

Tests still pass. Thanks for any review.

[erilem]

Andreas, thanks for the patch, it looks good. Personally I'd make it so the factor value is computed the first time it is needed as opposed to at script evaluation time.

[erilem]

Another comment: before commit could you comment in the code where the 320 value comes from? Thanks.

[ahocevar]

calculate factor the first time it is used; added comment about 320

[ahocevar]

(In [11868]) working around drawImage issue on Android 2.1 devices. r=erilem (closes #3243)