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/HelloIndices.cpp

@@ -24,6 +24,8 @@ nxt::Buffer vertexBuffer;
 
 nxt::Queue queue;
 nxt::Pipeline pipeline;
+nxt::RenderPass renderpass;
+nxt::Framebuffer framebuffer;
 
 void initBuffers() {
     static const uint32_t indexData[3] = {
@@ -82,7 +84,9 @@ void init() {
         .SetInput(0, 4 * sizeof(float), nxt::InputStepMode::Vertex)
         .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)
@@ -92,10 +96,12 @@ void init() {
 void frame() {
     static const uint32_t vertexBufferOffsets[1] = {0};
     nxt::CommandBuffer commands = device.CreateCommandBufferBuilder()
-        .SetPipeline(pipeline)
-        .SetVertexBuffers(0, 1, &vertexBuffer, vertexBufferOffsets)
-        .SetIndexBuffer(indexBuffer, 0, nxt::IndexFormat::Uint32)
-        .DrawElements(3, 1, 0, 0)
+        .BeginRenderPass(renderpass, framebuffer)
+            .SetPipeline(pipeline)
+            .SetVertexBuffers(0, 1, &vertexBuffer, vertexBufferOffsets)
+            .SetIndexBuffer(indexBuffer, 0, nxt::IndexFormat::Uint32)
+            .DrawElements(3, 1, 0, 0)
+        .EndRenderPass()
         .GetResult();
 
     queue.Submit(1, &commands);