Adjust texture data and threshold

Adjust texture data and threshold

Affects:
dEQP-GLES3.functional.texture.wrap.*
dEQP-GLES2.functional.texture.wrap.*

Components: OpenGL

VK-GL-CTS issue: 2589

Root cause:
We treat all variable which have relaxed precision decoration as half precision value.
1. When run in repeat&linear mode, sampling texture edge will get an
unexpect result.
2. When run in nearest mode, threshold is 2/256, but texture gradient is
2/255. so, if target texel's deviation more then 1, will get a fail
result.

Solution:
1. Adjust texture data, make the difference between two adjacent texel under 2/256
in per-channel.
2. Adjust threshold to 2/255 in RGBA_8888 mode, adjust threshold to (1 + 256/16) in RGBA_4444 mode.
3. Adjust texture data, make the texture gradient like a pyramid in
repeat mode.

Change-Id: I2382d36bdeb227e81d0d0b8f76f61d26a00f0fcd