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

src/backend/Texture.cpp

@@ -19,7 +19,7 @@
 
 namespace backend {
 
-    size_t TextureFormatPixelSize(nxt::TextureFormat format) {
+    uint32_t TextureFormatPixelSize(nxt::TextureFormat format) {
         switch (format) {
             case nxt::TextureFormat::R8G8B8A8Unorm:
                 return 4;