When the visible viewport rect changes, we walk the layout tree and
check where each LayoutImage is in relation to the viewport rect.
Images outside have their bitmaps marked as volatile.
Note that the bitmaps are managed by ImageDecoder objects. If a bitmap
is purged by the kernel while volatile, we construct a new ImageDecoder
next time we need pixels for the image.
We now wait until the pixels are actually needed before fully decoding
images in <img> elements.
This needs some more work and is currently a bit memory-wasteful since
we'll hang on to the raw image data forever.
LayoutReplaced objects can now participate in inline layout.
It's very hackish, but basically LayoutReplaced will just add itself to
the last line in the containing block.
This patch gets rid of the idea that only LayoutInline subclasses can
be split into lines, by moving the split_into_lines() virtual up to
LayoutNode and overriding it in LayoutReplaced.
This patch adds parsing of <img> into HTMLImageElement objects.
It also adds LayoutImage and its parent class LayoutReplaced, which is
going to represent CSS "replaced elements."
