Always use GLSL ES 100 shaders in swift shaders

Summary:
OpenGL ES 3 on rainbow has a bug, which originates from a bug in swiftshaders.

the issue is that swiftshaders is returning -1 when we use
```
glGetActiveUniformsiv(,..,GL_UNIFORM_OFFSET, ...)
```
to query the offset of a member in a uniform buffer. We believe this is a swift shaders bug, so we work around this by always using GLSL es 100 shaders on rainbow

Reviewed By: dmannemela, syeh1

Differential Revision: D49329247

fbshipit-source-id: 54a8245704d932e7984e7521bef9d28367d1e416

src/igl/opengl/Version.cpp

@@ -223,6 +223,13 @@ GLVersion getGLVersion(const char* version, bool constrain) {
 }
 
 ShaderVersion getShaderVersion(GLVersion version) {
+#if IGL_PLATFORM_LINUX_SWIFTSHADER
+  // Swiftshader has a bug when it comes to uniform blocks. When calling
+  // `glGetActiveUniformsiv(,..,GL_UNIFORM_OFFSET, ...)`, it always returns -1. We work around this
+  // by forcing GLSL ES 100 shaders in swiftshaders.
+  // TODO(dunfanlu): consider patching swift shaders to fix this.
+  return {ShaderFamily::GlslEs, 1, 0};
+#else
   // TODO: Return proper GLSL ES versions
   switch (version) {
   case GLVersion::v2_0_ES:
@@ -263,6 +270,7 @@ ShaderVersion getShaderVersion(GLVersion version) {
     IGL_ASSERT_NOT_REACHED();
     return {};
   }
+#endif
 }
 
 std::string getStringFromShaderVersion(ShaderVersion version) {