Vulkan: generateMipmaps for 2D textures - This implements cpu-only mipmaps generation. To do in the next CLs: - Cube textures support - GPU faster path with vkCmdBlitImage - Find the bug with clearing of level 0 mips. - Investigate the intel linux specific bug. Bug: angleproject:2502 Change-Id: Iecd73cb45ebc9c79ce6291c4529eb27330b51e2c Reviewed-on: https://chromium-review.googlesource.com/1064194 Commit-Queue: Luc Ferron <lucferron@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>