SGB border transfer doesn't respect BGP

[pinobatch]

This week, I released the first version of TRN Stress, an acceptance test ROM for loading Super Game Boy borders. mGBA passed all but two.

On 2024-05-08, endrift and beware were discussing emulation of a Game Boy Color game including Super Game Boy borders that were loading wrong because it expected objects not to be included in the transfer. This gave me the idea to build some acceptance tests for SGBC border loading as a step toward defining what SGBC even is. (Compare #2669, #2832.) But first, I needed acceptance tests for existing SGB transfers. I looked around for inspiration and found plans that I had drawn up in fourth quarter 2022. That led to TRN Stress, a successor to SGBears from September 2021.

Environment:

$ mgba-qt --version; lsb_release -a | grep Description
mgba-qt 0.11-779-b7729c9 (b7729c9e80736e34a46ff9f38fcaf0d1ed97a7b2)
Description:	Ubuntu 23.10

Steps to reproduce:

1. Download, unzip, and open TRN Stress, either the MBC5 version or the 32K version.
2. If prompted, switch to Super Game Boy model in Tools > Settings > Game Boy.
3. Select each of scenes 1 through 16. Look at as many as you can bear.

Expect: Each displays a scene that's not obviously corrupt. Scene 4 "2bpp via BGP" should look like this ad for Lowney's cocoa.

Actual: Scene 2 "Inverted BGP" doesn't load at all, and scene 4 has diagonal dirt all over the image.