- Because of the difference in the SNORM to FLOAT conversion formula
  between GLES 2.0 and 3.0, OES_vertex_type_10_10_10_2 is disabled
  when the context version is lower than 3.0.
- Modify test conversion formula to be compliant with GLES 3.0 equation

Bug: angleproject:3868
Test: angle_end2end_tests --gtest_filter=VertexAttributeTest*Packed*Vulkan
Change-Id: I8b85d8146ee05353bc40fa0022f05d6634c33110
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1792197
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>