Evas GL common: Disable evas gl preload by default
Unfortunately, this "feature" has many problems and does not really
fix those it was supposed to address:
- Elm Photocam becomes horrible to use (the transition from low-res to high-res tiles triggers this miniature path).
- Evas async preload callback is called before the full image is ready (ie. the texture is not uploaded yet), when really the preload callback should be triggered only once the image is 100% ready. (TODO)
- Sometimes the miniature image keeps being used even though the main image has been uploaded (eg. with E background). Maybe the object image is not redrawn when it should.
- This uses a separate thread for the upload, which is both a good and bad idea because we need to do a make current. Also, this does not upload the full-res image tile by tile, but only in one pass, thus blocking the render loop until finished.
This patch changes the env var from "EVAS_GL_NOPRELOAD" to
"EVAS_GL_PRELOAD" (and only "1" will enable).
Sorry Cedric, we can talk later about how to improve this.