Increase subPixelPrecisionBits from 4 to 8

This is a requirement of the Vulkan Roadmap 2022 profile. We've long
kept it at 4 sub-pixel precision bits because a significant number of
mobile devices report this number.

Test: dEQP-VK.api.info.profiles.roadmap_2022
Bug: b/226657516
Change-Id: Ifd95f44658590ff69ffe6d742186e3806c38bbfa
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/69451
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>

src/Vulkan/VkConfig.hpp

diff --git a/src/Vulkan/VkConfig.hpp b/src/Vulkan/VkConfig.hpp
index 4884986..ca2f5ce 100644
--- a/src/Vulkan/VkConfig.hpp
+++ b/src/Vulkan/VkConfig.hpp
@@ -90,7 +90,7 @@
 
 constexpr int MAX_SAMPLER_ALLOCATION_COUNT = 4000;
 
-constexpr int SUBPIXEL_PRECISION_BITS = 4;
+constexpr int SUBPIXEL_PRECISION_BITS = 8;
 constexpr float SUBPIXEL_PRECISION_FACTOR = static_cast<float>(1 << SUBPIXEL_PRECISION_BITS);
 constexpr int SUBPIXEL_PRECISION_MASK = 0xFFFFFFFF >> (32 - SUBPIXEL_PRECISION_BITS);