Support object-fit, object-position and aspect-ratio #604

Part of #552 (step 28, missing support, medium priority).

object-fit, object-position and aspect-ratio are never read.

• An image given both a width and a height is stretched to that box, so logos and product photos in fixed-size slots come out distorted.
• A box given aspect-ratio keeps its content height.

Measured on gravitypdf at 60b82630, PHP 8.5.4, from the image's cm matrix and the filled rectangles. The image is tests/data/img/bg.jpg, about 1 : 1.16.

Cause.

• Img::open() reads WIDTH, HEIGHT and the min-/max- values (src/Tag/Img.php:136-141). ImageSizing::fit() (src/Image/ImageSizing.php:26, called at src/Tag/Img.php:274) fills in whichever is missing from the image's ratio. When both are given, the image is drawn at exactly that size, with nothing to crop or letterbox it.
• Blocks get their height from their content, or from height.

Fix.

• object-fit on img (and svg drawn as an image):

  • fill is today's behaviour
  • contain scales the image to fit inside the box
  • cover scales it to cover the box
  • none keeps the natural size
  • scale-down is the smaller of none and contain

  The box stays the size width and height give it. The image is drawn at the fitted size and clipped to the content box with re W n, inside the q … Q the image already has.

• object-position. Place the fitted image in the box: keywords, % and lengths, as background-position is read. The default is 50% 50%.

• aspect-ratio on img. When one of width and height is given, derive the other from the ratio, not from the image; auto 3/2 means the image's own ratio when it has one.

• aspect-ratio on a block. When height is auto and the width is known, set the height from the ratio. Content taller than that grows the box: CSS's automatic minimum size, with min-height: auto. That is how mPDF treats css_set_height today. Keep it that way for aspect-ratio when A percentage height on a block resolves against the page area, and height acts as a minimum that overflow: hidden does not clip #587 makes height exact.

• Borders, padding and radius go around the box, not the fitted image, and a radius clips the image.

Tests. Each object-fit value on a wide and a tall image, object-position with keywords, % and lengths, aspect-ratio with only a width, only a height, and neither, a rounded image with cover, and an image in a table cell.