Introduce render passes

* First API design (many features missing, including input attachments)
* Metal implementation (no OpenGL yet)
* Render-to-texture demo (a little broken until we have depth buffers)
* Update examples to use render passes

examples/HelloInstancing.cpp

@@ -24,6 +24,8 @@ nxt::Buffer instanceBuffer;
 
 nxt::Queue queue;
 nxt::Pipeline pipeline;
+nxt::RenderPass renderpass;
+nxt::Framebuffer framebuffer;
 
 void initBuffers() {
     static const float vertexData[12] = {
@@ -89,7 +91,9 @@ void init() {
         .SetInput(1, 2 * sizeof(float), nxt::InputStepMode::Instance)
         .GetResult();
 
+    CreateDefaultRenderPass(device, &renderpass, &framebuffer);
     pipeline = device.CreatePipelineBuilder()
+        .SetSubpass(renderpass, 0)
         .SetStage(nxt::ShaderStage::Vertex, vsModule, "main")
         .SetStage(nxt::ShaderStage::Fragment, fsModule, "main")
         .SetInputState(inputState)
@@ -99,10 +103,12 @@ void init() {
 void frame() {
     static const uint32_t vertexBufferOffsets[1] = {0};
     nxt::CommandBuffer commands = device.CreateCommandBufferBuilder()
-        .SetPipeline(pipeline)
-        .SetVertexBuffers(0, 1, &vertexBuffer, vertexBufferOffsets)
-        .SetVertexBuffers(1, 1, &instanceBuffer, vertexBufferOffsets)
-        .DrawArrays(3, 4, 0, 0)
+        .BeginRenderPass(renderpass, framebuffer)
+            .SetPipeline(pipeline)
+            .SetVertexBuffers(0, 1, &vertexBuffer, vertexBufferOffsets)
+            .SetVertexBuffers(1, 1, &instanceBuffer, vertexBufferOffsets)
+            .DrawArrays(3, 4, 0, 0)
+        .EndRenderPass()
         .GetResult();
 
     queue.Submit(1, &commands);