clang/gcc: enable a bunch more warnings (#91)

* clang/gcc: enable -pedantic warnings

* suppress a GCC-specific warning in stb_image

* And some clang-specific warnings

* -Wconversion (clang) -Wold-style-cast (clang+gcc)

and fix a few warnings that show up with these (and a few more with
-Wconversion on gcc, even though that's not enabled by default)

* bunch more warnings

* fixes

* remove merge error

examples/Animometer.cpp

@@ -130,7 +130,7 @@ void frame() {
     size_t i = 0;
 
     std::vector<nxt::CommandBuffer> commands(50);
-    for (int j = 0; j < 50; j++) {
+    for (size_t j = 0; j < 50; j++) {
 
         nxt::CommandBufferBuilder builder = device.CreateCommandBufferBuilder()
             .BeginRenderPass(renderpass, framebuffer)