Sample from texel centers in downchunk shaders

Previously we weren't sampling from texel centers, which caused
incorrect pixels to be sampled when using nearest neighbor sampling.

Bug: 329464641
Flag: com.android.graphics.surfaceflinger.flags.local_tonemap_screenshots
Test: librenderengine_test
Test: Screenshots of UltraHDR and HDR video
Change-Id: Id1e3ea070368aecb18e5bd1ae7093537c76f6ddf