Improve portability of samples in tests/cases: use supported vertex formats

E.g. draw_triangle_list.amber uses 3-channel 8bits-per-channel UNORM :

```
BUFFER vert_color DATA_TYPE R8G8B8_UNORM DATA
255 0 0
255 0 0
255 0 0
...
```

But, for example, Vulkan does not require R8G8B8_UNORM to be supported as a vertex format.
See the checkmarks for VK_FORMAT_FEATURE_VERTEX_BUFFER_BIT in the table "Mandatory format support: 1-3 byte-sized channels". VK_FORMAT_R8G8B8_UNORM has no checkmark.

So change that sample to a 4-channel format with same channel width and also UNORM.
The next table "Mandatory format support: 4 byte-sized channels" shows that VK_FORMAT_R8G8B8A8_UNORM is required to be supported.

Review the other samples and update them to use only required vertext formats. This will improve portability of the test case.s