gsk: Remove unnecessary G_GNUC_FALLTHROUGH

In GSK the following pattern is used four times:
```
  switch (self->filter)
    {
      default:
        g_assert_not_reached ();
        G_GNUC_FALLTHROUGH;
      case GSK_GPU_BLIT_LINEAR:
        filter = GL_LINEAR;
        break;

      case GSK_GPU_BLIT_NEAREST:
        filter = GL_NEAREST;
        break;
    }
```
The G_GNUC_FALLTHROUGH macro is not required. When G_DISABLE_ASSERT
is defined the body of the `default` case is empty, thus there is
no need. When G_DISABLE_ASSERT is not defined the body of the `default`
case contains g_assert_not_reached() thus it won't fallthrough.

This resolves the following:
```
[221/1379] Compiling C object gsk/libgsk.a.p/gpu_gskgpublitop.c.o
[...]
error: fallthrough annotation in unreachable code [-Werror,-Wimplicit-fallthrough]
1 error generated.
```

gsk/gpu/gskgpublitop.c

@@ -85,7 +85,6 @@ gsk_gpu_blit_op_vk_command (GskGpuOp              *op,
     {
       default:
         g_assert_not_reached ();
-        G_GNUC_FALLTHROUGH;
       case GSK_GPU_BLIT_LINEAR:
         filter = VK_FILTER_LINEAR;
         break;
@@ -160,7 +159,6 @@ gsk_gpu_blit_op_gl_command (GskGpuOp          *op,
     {
       default:
         g_assert_not_reached ();
-        G_GNUC_FALLTHROUGH;
       case GSK_GPU_BLIT_LINEAR:
         filter = GL_LINEAR;
         break;

gsk/gpu/gskgpunodeprocessor.c

@@ -563,7 +563,6 @@ extract_scale_from_transform (GskTransform *transform,
     {
     default:
       g_assert_not_reached ();
-      G_GNUC_FALLTHROUGH;
     case GSK_TRANSFORM_CATEGORY_IDENTITY:
     case GSK_TRANSFORM_CATEGORY_2D_TRANSLATE:
       *out_scale_x = 1.0f;

gsk/gpu/gskgpurenderpassop.c

@@ -54,7 +54,6 @@ gsk_gpu_render_pass_type_to_vk_image_layout (GskRenderPassType type)
   {
     default:
       g_assert_not_reached ();
-      G_GNUC_FALLTHROUGH;
     case GSK_RENDER_PASS_PRESENT:
       return VK_IMAGE_LAYOUT_PRESENT_SRC_KHR;
     case GSK_RENDER_PASS_OFFSCREEN: