Improve memory usage for image import and PortableCompressedTexture2D

When importing images, we store a compressed version of the image to a .ctex file with ResourceImporterTexture::save_to_ctex_format.  When importing many large images at once, this can use a large amount of memory, especially when the .ctex file uses WebP format.

This change is for ResourceImporterTexture::save_to_ctex_format to use the original Image object instead of p_image->get_image_from_mipmap(0), to avoid creating a copy of the full uncompressed image when looping through the base Image and mipmaps.  This reduces the import memory usage for large images by around 10% when using WebP, and 35-40% when Project Settings/Rendering/Textures/Lossless Compression/Force PNG is enabled, may vary depending on the image and number of import threads running.  Same change applied to PortableCompressedTexture2D::create_from_image, which has similar logic.

This helps with #92084, but does not fully resolve the issue on its own, as compressing with WebP on many threads can still use a large amount of memory - this just lowers that amount, and makes it more likely that enabling "Force PNG" will reduce memory usage enough to import the files.

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