GPU mipmap generation broken when running with SwiftShader

Step 1: enable swiftshader
https://github.com/google/bigwheels/blob/main/docs/software_rendering.md

Step 2: run a sample
for example: 05_cube_textured
or to see more clearly: run vk_27_mipmap_demo, select a different mip level (> 0) to be generated using GPU


Background:
Mipmap creation code in https://github.com/google/bigwheels/blob/main/src/ppx/graphics_util.cpp#L273