Before this commit, the following sequence would cause an assert:

    glCompressedTexImage2D(GL_TEXTURE_2D, 0,
                           GL_COMPRESSED_RGBA_ASTC_8x8, ...);
    glCompressedTexImage2D(GL_TEXTURE_2D, 1,
                           GL_COMPRESSED_RGBA_ASTC_8x8, ...);

This is due to code we added that clears the compressed texture cache
when it detects we already have an entry for the texture level. This is
allowed in GL, to respecify a texture level by simply calling
glCompressedTexImage on it again.

The problem is we would clear the entire texture from the cache, not
just the level being respecified.

Test: Recapture Asphalt 8
Bug: angleproject:5678
Change-Id: I473d6ea1cb8823bfec680ddd78c3457b93ecc431
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2716639Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>