Evas GL common: Disable evas gl preload by default

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.