PNG screendump has R/B channels swapped

Qemu 7.2, aarch64 system emulation on both aarch64 or x86_64 hosts.

With qemu 7.2.0 a screenshot taken via the QMP screendump command is correct when output as a PPM, but when I write a PNG the red and blue channels are swapped.

First, the file contains a redundant alpha channel, so the PNG could be written as PNG_COLOR_TYPE_RGB instead of PNG_COLOR_TYPE_RGB_ALPHA.

Second, the pixel data needs to be in network byte order. I note that the PPM export code uses a PIXMAN_BE_r8g8b8 pixman buffer for the rows, whereas the PNG code uses PIXMAN_a8r8g8b8. As we don't need the alpha channel anymore then using PIXMAN_BE_r8g8b8 should resolve the issue.