Fix two uninitialized variables

Found when running in valgrind looking at another issue.

- RenderPass' depth_stencil_target
    Tripped in SDL_BindGPUFragmentSamplers when not binding a DS target

- VulkanCommandBuffer's swapchainRequested
    Tripped in VULKAN_Submit for the end transition barrier when
    creating an image. The field is only reset when reused, not on first
    use

(cherry picked from commit 265236d952)

src/gpu/SDL_gpu.c

@@ -1785,6 +1785,8 @@ SDL_GPURenderPass *SDL_BeginGPURenderPass(
         commandBufferHeader->render_pass.num_color_targets = num_color_targets;
         if (depth_stencil_target_info != NULL) {
             commandBufferHeader->render_pass.depth_stencil_target = depth_stencil_target_info->texture;
+        } else {
+            commandBufferHeader->render_pass.depth_stencil_target = NULL;
         }
     }
 

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -9408,6 +9408,8 @@ static bool VULKAN_INTERNAL_AllocateCommandBuffer(
     commandBuffer->usedUniformBuffers = SDL_malloc(
         commandBuffer->usedUniformBufferCapacity * sizeof(VulkanUniformBuffer *));
 
+    commandBuffer->swapchainRequested = false;
+
     // Pool it!
 
     vulkanCommandPool->inactiveCommandBuffers[vulkanCommandPool->inactiveCommandBufferCount] = commandBuffer;