linux: fix getting the initial content size (#7604)

Fix found by @h3mosphere (thanks!)

Solves the Vulkan validation on start on some platforms about the
mismatched surface size, e.g.
```
VUID-VkSwapchainCreateInfoKHR-imageExtent-01274(ERROR / SPEC): msgNum: 2094043421 - Validation Error: [ VUID-VkSwapchainCreateInfoKHR-imageExtent-01274 ] Object 0: handle = 0x55dff99554c0, type = VK_OBJECT_TYPE_DEVICE; | MessageID = 0x7cd0911d | vkCreateSwapchainKHR() called with imageExtent = (1920,1080), which is outside the bounds returned by vkGetPhysicalDeviceSurfaceCapabilitiesKHR(): currentExtent = (1920,1016), minImageExtent = (1920,1016), maxImageExtent = (1920,1016). The Vulkan spec states: imageExtent must be between minImageExtent and maxImageExtent, inclusive, where minImageExtent and maxImageExtent are members of the VkSurfaceCapabilitiesKHR structure returned by vkGetPhysicalDeviceSurfaceCapabilitiesKHR for the surface (https://www.khronos.org/registry/vulkan/specs/1.3-extensions/html/vkspec.html#VUID-VkSwapchainCreateInfoKHR-imageExtent-01274)
```

Release Notes:
- N/A

Co-authored-by: Mikayla Maki <mikayla@zed.dev>