Fix texture resource reload bug

[aqnuep]

If a non-imported texture resource file (e.g. DDS) gets updated the editor
doesn't reload it. The cause of the problem is two-fold:

First, the code of ImageTexture assumes that textures are always imported
from an image, but that's not the case for e.g. DDS. This change thus adds
code to issue a resource reload in case an image reload is not possible
(which is the case for non-imported texture resources).

Second, the code is filled with bogus calls to Image::get_image_data_size()
to determine the mipmap offset when that should be done using
Image::get_image_mipmap_offset(). Previous code literally passed the integer
mip level value to Image::get_image_data_size() where that actually expects
a boolean. Thus this part of the change might actually solve some other
issues as well.

To be pedantic, the texture_get_data() funciton of the rasterizer drivers is
still quite a mess, as it only ever returns the whole mipchain when
GLES_OVER_GL is set (practically only on desktop builds) but this change does
not attempt to resolve that.

[akien-mga]

Thanks!

@hpvb Should be fine to cherrypick for 3.1 according to @reduz.

[akien-mga]

Cherry-picked for 3.1.2.