allow multiple image reload attempts before going pink

[tschaub]

The following patch takes Stuart Eve's idea to avoid all the pink tiles. By default, OL would only try once. If a user wanted to try multiple image requests before going pink, they would set OpenLayers.IMAGE_RELOAD_ATTEMPTS = 3 (or something similar).

This is somewhat tough to test (requires a timeout), but I have seen it working with the vmap0 wms by adding a console.log, watching additional requests, and getting no pink. (With 0 reload attempts, I frequently get pink.)

Please review.

[tschaub]

patch to allow multiple image reloads before going pink

[crschmidt]

Applied with r2048. Thanks Tim/Stuart/Chris.