In generateMipmaps, remove hard coded depth of 1, shift depth
like every other dimension.

Remove MipmapsForTexture3D test pixel value check, in es spec3.2 8.14.4
"No particular filter algorithm is required, though a box filter is recommended."
It's implementation-dependent. In current angle implementation, will
choose VK_FILTER_LINEAR/VK_FILTER_NEAREST according to vkdeice support.

Bug: angleproject:3983
Test: MipmapTestES3.MipmapsForTexture3D/ES3_Vulkan
Test: dEQP-GLES3.functional.samplers.single_tex_3d.diff_max_lod
Test: dEQP-GLES3.functional.samplers.single_tex_3d.diff_min_lod
Test: dEQP-GLES3.functional.samplers.multi_tex_3d.diff_max_lod
Test: dEQP-GLES3.functional.samplers.multi_tex_3d.diff_min_lod
Change-Id: I5e73f8c743053aeb521b5e0b3e372bbe77e57ad2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2076740Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>