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/HelloCompute.cpp

@@ -35,7 +35,7 @@ void init() {
     queue = device.CreateQueueBuilder().GetResult();
 
     struct {uint32_t a; float b;} s;
-    memset(&s, sizeof(s), 0);
+    memset(&s, 0, sizeof(s));
     buffer = device.CreateBufferBuilder()
         .SetAllowedUsage(nxt::BufferUsageBit::TransferDst | nxt::BufferUsageBit::Uniform | nxt::BufferUsageBit::Storage)
         .SetInitialUsage(nxt::BufferUsageBit::TransferDst)