Validation layers revealed 9965 VkSamplers allocated against a device
limit of 4000 — every VkTexture created its own sampler even when
configurations were identical. This exhausted NVIDIA's sampler pool
and caused intermittent SIGSEGV in vkCmdBeginRenderPass.
Add a thread-safe sampler cache in VkContext that deduplicates samplers
by FNV-1a hash of all 14 VkSamplerCreateInfo fields. All texture,
render target, renderer, water, and loading screen sampler creation
now goes through getOrCreateSampler(). Textures set ownsSampler_=false
so shared samplers aren't double-freed.
Also auto-disable anisotropy in the cache when the physical device
doesn't support the samplerAnisotropy feature, fixing the validation
error VUID-VkSamplerCreateInfo-anisotropyEnable-01070.
