fix: letterbox image to exact target size when fitscreen is false

[kjames2001]

Problem

When push_image is called with fitscreen false (the default) and the
source image's aspect ratio differs from the requested width/height,
image_to_rgb565 uses PIL.Image.thumbnail(), which preserves aspect
ratio and returns an image smaller than the requested box.

Example: a square 512x512 cover pushed into a 400x160 object is
thumbnail-scaled to 160x160, not 400x160.

The plate decodes the RGB565 payload using the width stored in the
header as the row stride. A pushed image narrower than the object box
therefore renders doubled / split (the top of the image repeated,
second copy cut off — a "1.5 image") instead of centered. 16:9 sources
into 16:9 boxes are unaffected, which is why cameras usually look fine
while square covers glitch.

Fix

After thumbnail(), paste the result onto a black canvas at the exact
target size (centered). The pushed width now always matches the object
width, which is what the plate's decoder expects.

• Sources smaller than the box keep the existing no-upscale behaviour
  (the min() clamp is unchanged; output identical to before).
• The fitscreen=True path (ImageOps.fit) is untouched.

Verification

• Functional test of image_to_rgb565 across: square->wide box,
  16:9->16:9 box (196x110 / 400x160 / 480x270), small source, exact
  size, RGBA source, and the fitscreen path — all outputs carry the
  expected width/height header.
• Verified on a real 0.7.9 installation (openHASP 0.7.0-rc12 plate,
  480x480): the doubled image no longer appears; the rendered box is
  pixel-identical to the expected letterboxed frame (max channel diff 5).
• black --check clean.