Replace all glGenTextures/glTexImage2D calls in UI code with Vulkan texture
uploads via new VkContext::uploadImGuiTexture() helper. This fixes segfaults
from calling OpenGL functions without a GL context (null GLEW function pointers).
- Add uploadImGuiTexture() to VkContext with staging buffer upload pattern
- Convert game_screen, inventory_screen, spellbook_screen, talent_screen
from GLuint/GL calls to VkDescriptorSet/Vulkan uploads
- Fix loading_screen clearValueCount (was 1, needs 2 or 3 for MSAA)
- Add error handling: revert to 1x if recreateSwapchain fails
- Clamp requested MSAA to device maximum before applying
- Retry MSAA color image allocation without TRANSIENT on failure
- Remove redundant vkDeviceWaitIdle from WMO/M2/Character recreatePipelines
(caller already waits once, was causing ~13 stalls instead of 1)
