Tried a Radeon HD 6450 on my PCIe PowerMac G5 11,2 for another comparison.
Graphics are rendered correctly! So it seems this bug is a specific r300 driver
problem, not a generic Big Endian problem.

$ glxinfo | grep -i opengl
OpenGL vendor string: X.Org
OpenGL renderer string: AMD CAICOS (DRM 2.49.0 / 4.9.85-gentoo)
OpenGL core profile version string: 3.2 (Core Profile) Mesa 18.0.0-rc4
OpenGL core profile shading language version string: 1.50
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile
OpenGL core profile extensions:
OpenGL version string: 3.0 Mesa 18.0.0-rc4
OpenGL shading language version string: 1.30
OpenGL context flags: (none)
OpenGL extensions:
OpenGL ES profile version string: OpenGL ES 3.1 Mesa 18.0.0-rc4
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10
OpenGL ES profile extensions: