Add desktop OpenGL support for GL_EXT_fragment_shading_rate

[Wang-JingWen]

The changes include:

• Add desktop OpenGL support.

  • Rebased the section numbers and page numbers against GL 4.6 core.
  • Moved the section [[If OVR_Multiview is supported]] from GL 4.6 main context to section Interactions with OVR_multiview.
  • ES 3.2 has no multiple viewport support, whereas GL 4.6 supports. Hence, added a new token FRAGMENT_SHADING_RATE_PRIMITIVE_RATE_WITH_MULTI_VIEWPORT_SUPPORTED_EXT and multiple viewport references to Section 14.X.2.

• Refine some ambiguous or verbose descriptions.

  • Refined the definition for <texelWidth> and <texelHeight> in Section 14.X.3.
  • Clarified the defination of _effective shading rate_ in Section 14.X.4.
  • Refined the definition for <combinerOp0> and <combinerOp1> in Section 14.X.4.
  • The paragraph "If the final combined rate is one of the rates ......" in Section 14.X.4 is imprecise, made it adheres to VK_KHR_fragment_shading_rate specification.

• Polish the extension formatting.

  • Removed the unexpected indent (the first 4 blank spaces) for the whole file.
  • Limited the lines to 80 characters wide (not exactly, tables excepted).
  • The number of preceding spaces for each sentence was inconsistent(1 space and 2 spaces mixed). Uniformed the format to 1 space preceding.
  • Moved sections Interactions with * prior to Errors, which is more in line with norms.

[janharaldfredriksen-arm]

Generally looks good to me.

[janharaldfredriksen-arm]

We need some statement somewhere to say that this is not available when implemented against GLES.
Maybe something like:

Dependencies on OpenGL 4.6

If OpenGL 4.6 is not supported, references to the FRAGMENT_SHADING_RATE_PRIMITIVE_RATE_WITH_MULTI_VIEWPORT_SUPPORTED_EXT should
be removed.

[Wang-JingWen]

Nice hint! Added in section Interactions with OpenGL ES 3.2

Interactions with OpenGL ES 3.2

    If implemented in OpenGL ES, references to multiple viewports and
    FRAGMENT_SHADING_RATE_PRIMITIVE_RATE_WITH_MULTI_VIEWPORT_SUPPORTED_EXT
    should be removed.