ES2 support: filament and filamat changes

This CL contains two parts:
- changes to matc/filamat
- changes to filament itself

Filamat can now generate ES2 compatible shaders. Only the unlit variant
is supported. Fog and picking are supported as well.
post-processing, skinning, instancing, all lighting and shadowing are not supported.

Filament is updated to not issue commands that are not supported in ES2.

Addtionnally, the hello-triangle sample is updated to work on an ES2 device.

libs/filamat/src/GLSLPostProcessor.cpp

@@ -524,6 +524,12 @@ void GLSLPostProcessor::fullOptimization(const TShader& tShader,
         glslOptions.fragment.default_int_precision = glslOptions.es ?
                 CompilerGLSL::Options::Precision::Mediump : CompilerGLSL::Options::Precision::Highp;
 
+        // TODO: this should be done only on the "feature level 0" variant
+        if (config.featureLevel == 0) {
+            // convert UBOs to plain uniforms if we're at feature level 0
+            glslOptions.emit_uniform_buffer_as_plain_uniforms = true;
+        }
+
         CompilerGLSL glslCompiler(std::move(spirv));
         glslCompiler.set_common_options(glslOptions);