Initial support for compute

* minimal backend support for compute
- added api to dispatch a compute shader
- added api to create and bind a ssbo
- added api to read back a buffer
Only implemented in the gl backend

* Add a backend compute test suite
* basic support for compute shaders in matc
this is still very much work-in-progress.
We're not supporting images nor ssbo for now.

* rename UniformInterfaceBlock to BufferInterfaceBlock
* augment BufferInterfaceBlock to support ssbo features
- add support for std430
- add support for ssbo
- add support for variable-size array
- add support for memory qualifiers

* reformat MaterialBuilder
* material format: move subpasses outside of parameters
subpasses now are their own json property instead of being a
"parameter".

* refactor parameter() methods to match Buffer/SamplerInterfaceBlock
We're just shuffling the arguments.

* add support for buffers in .mat files
* filamat now generates buffer blocks (ssbo)
* take feature level into consideration when optimizing shaders
* don't store the 'uniform binding' chunk for level 2 materials
this includes some refactoring/cleanups of MaterialParser

* matinfo: fixes for compute
- separate subpasses from parameters
- don't attempt to print material properties

samples/sample_cloth.cpp

@@ -200,16 +200,17 @@ static void setup(Engine* engine, View* view, Scene* scene) {
     }
 
     MaterialBuilder::init();
-    MaterialBuilder builder = MaterialBuilder()
+    MaterialBuilder builder;
+    builder
             .name("DefaultMaterial")
             .targetApi(MaterialBuilder::TargetApi::ALL)
 #ifndef NDEBUG
             .optimization(MaterialBuilderBase::Optimization::NONE)
 #endif
             .require(VertexAttribute::UV0)
-            .parameter(MaterialBuilder::SamplerType::SAMPLER_2D, "normalMap")
-            .parameter(MaterialBuilder::SamplerType::SAMPLER_2D, "basecolorMap")
-            .parameter(MaterialBuilder::SamplerType::SAMPLER_2D, "roughnessMap")
+            .parameter("normalMap", MaterialBuilder::SamplerType::SAMPLER_2D)
+            .parameter("basecolorMap", MaterialBuilder::SamplerType::SAMPLER_2D)
+            .parameter("roughnessMap", MaterialBuilder::SamplerType::SAMPLER_2D)
             .material(R"SHADER(
                 void material(inout MaterialInputs material) {
                     vec2 uv = getUV0() * 2.0;