lavc/vulkan: add a u8vec2buf buffer type

Useful, since it doesn't have alignment limitations.

libavcodec/vulkan/common.comp

@@ -22,6 +22,10 @@ layout(buffer_reference, buffer_reference_align = 1) buffer u8buf {
     uint8_t v;
 };
 
+layout(buffer_reference, buffer_reference_align = 1) buffer u8vec2buf {
+    u8vec2 v;
+};
+
 layout(buffer_reference, buffer_reference_align = 2) buffer u16buf {
     uint16_t v;
 };