Skip clears with an empty scissor.
Fixes the following validation error:
00:00:09.734 [wlr] [render/vulkan/vulkan.c:61] Validation Error: [ VUID-vkCmdClearAttachments-rect-02682 ] Object 0: handle = 0x62600001b100, type = VK_OBJECT_TYPE_COMMAND_BUFFER; | MessageID = 0xadbd476f | CmdClearAttachments(): pRects[0].rect.extent.width is zero. The Vulkan spec states: The rect member of each element of pRects must have an extent.width greater than 0 (https://www.khronos.org/registry/vulkan/specs/1.3-extensions/html/vkspec.html#VUID-vkCmdClearAttachments-rect-02682) (VUID-vkCmdClearAttachments-rect-02682)