Overview
========

SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's
internal shading language. SkSL is, at its heart, a single standardized version
of GLSL which avoids all of the various version and dialect differences found
in GLSL "in the wild", but it does bring a few of its own changes to the table.

Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V
before handing it over to the graphics driver.


Differences from GLSL
=====================

* Precision modifiers are not used. 'float', 'int', and 'uint' are always high
  precision. New types 'half', 'short', and 'ushort' are medium precision (we
  do not use low precision).
* Vector types are named <base type><columns>, so float2 instead of vec2 and
  bool4 instead of bvec4
* Matrix types are named <base type><columns>x<rows>, so float2x3 instead of
  mat2x3 and double4x4 instead of dmat4
* "@if" and "@switch" are static versions of if and switch. They behave exactly
  the same as if and switch in all respects other than it being a compile-time
  error to use a non-constant expression as a test.
* GLSL caps can be referenced via the syntax 'sk_Caps.<name>', e.g.
  sk_Caps.canUseAnyFunctionInShader. The value will be a constant boolean or int,
  as appropriate. As SkSL supports constant folding and branch elimination, this
  means that an 'if' statement which statically queries a cap will collapse down
  to the chosen branch, meaning that:

    if (sk_Caps.externalTextureSupport)
        do_something();
    else
        do_something_else();

  will compile as if you had written either 'do_something();' or
  'do_something_else();', depending on whether that cap is enabled or not.
* no #version statement is required, and it will be ignored if present
* the output color is sk_FragColor (do not declare it)
* use sk_Position instead of gl_Position. sk_Position is in device coordinates
  rather than normalized coordinates.
* use sk_PointSize instead of gl_PointSize
* use sk_VertexID instead of gl_VertexID
* use sk_InstanceID instead of gl_InstanceID
* the fragment coordinate is sk_FragCoord, and is always relative to the upper
  left.
* use sk_Clockwise instead of gl_FrontFacing. This is always relative to an
  upper left origin.
* you do not need to include ".0" to make a number a float (meaning that
  "float2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would
  often have to be expressed "float2(x, y) * 4.0". There is no performance
  penalty for this, as the number is converted to a float at compile time)
* type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported
* creating a smaller vector from a larger vector (e.g. float2(float3(1))) is
  intentionally disallowed, as it is just a wordier way of performing a swizzle.
  Use swizzles instead.
* Swizzle components, in addition to the normal rgba / xyzw components, can also
  be LTRB (meaning "left/top/right/bottom", for when we store rectangles in
  vectors), and may also be the constants '0' or '1' to produce a constant 0 or
  1 in that channel instead of selecting anything from the source vector.
  foo.rgb1 is equivalent to float4(foo.rgb, 1).
* All texture functions are named "sample", e.g. sample(sampler2D, float3) is
  equivalent to GLSL's textureProj(sampler2D, float3).
* Functions support the 'inline' modifier, which causes the compiler to ignore
  its normal inlining heuristics and inline the function if at all possible
* some built-in functions and one or two rarely-used language features are not
  yet supported (sorry!)