Store mipmap levels in deferred texture image

When creating the deferred texture image, detect if using medium / high
quality. If so, generate and store mipmaps in the deferred texture
image.

When creating a texture from that be sure to read it back out.

BUG=578304
R=bsalomon@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2034933003

Review-Url: https://codereview.chromium.org/2034933003
5 files changed