B10G11R11_UFLOAT blending is required in order for SwANGLE
to expose GL_EXT_color_buffer_float.

In this cl:
- I added a minor readability improvement by storing
  "*Pointer<Int>(data + OFFSET(DrawData, colorPitchB[index]))"
  in a variable, "pitchB" since it's used in every case.
- Added a constant for B10G11R11 masking
- Added clamping in the floating point blending code for
  unsigned floating point formats
- Fixed the VK_FORMAT_B10G11R11_UFLOAT_PACK32 output format
  which now takes the coverage mask into account
- Included a minor followup cleanup in ShaderCore

Bug: b/146223877
Tests: dEQP-VK.*b10g11r11*
Change-Id: Ifb95f34a10cdbee9d185bc25feba0aeaca0d9e70
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39929Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>