Add support for input attachments in SkSL spirv

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5115

Change-Id: I3e03a465a10c9aff62491d0f6e71105d1b650dab
Reviewed-on: https://skia-review.googlesource.com/5115
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/src/sksl/sksl.include b/src/sksl/sksl.include
index 83c6aed..d7b6326 100644
--- a/src/sksl/sksl.include
+++ b/src/sksl/sksl.include
@@ -286,6 +286,15 @@
 float texture(sampler2DRectShadow sampler, vec3 P);
 float texture($gsamplerCubeArrayShadow sampler, vec4 P, float compare);
 */
+
+// Currently we do not support the generic types of loading subpassInput so we have some explicit
+// versions that we currently use
+vec4 subpassLoad(subpassInput subpass);
+vec4 subpassLoad(subpassInputMS subpass, int sample);
+/*
+$gvec4 subpassLoad(gsubpassInput subpass);
+$gvec4 subpassLoad(gsubpassInputMS subpass, int sample);
+*/
 )
 
 // split into multiple chunks, as MSVC++ complains if a single string is too long