[Impeller] Support impeller::MipFilter::kBase for sampling from the base mip level. #147259
Description
SkMipmapMode::kNone is denotes sampling from the base mip level. Impeller today can either sample from the nearest or use trilinear filtering. To support what Skia does, Impeller must add a new mip filter mode for sampling from the base.
In terms of implementation, Metal supports this via MTLSamplerMipFilterNotMipmapped and Vulkan supports this via VkSamplerCreateInfo::maxLod set to 0.