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/HelloTriangle.cpp

@@ -63,7 +63,7 @@ void initTextures() {
     // Initialize the texture with arbitrary data until we can load images
     std::vector<uint8_t> data(4 * 1024 * 1024, 0);
     for (size_t i = 0; i < data.size(); ++i) {
-        data[i] = i % 253;
+        data[i] = static_cast<uint8_t>(i % 253);
     }