Previously Image::getMipLevelExtent() was the only way to retrieve the
extent of an image, which didn't make sense for images for which we're
not accessing the mipmaps, like framebuffer attachments. A getExtent()
method was added for these cases where we only need the full extent.

Bug: b/162315264
Change-Id: I9c2b8ed42cbbd573e5a08f4bfafc4653ad377f7f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47928
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>