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/tests/unittests/MathTests.cpp

@@ -64,7 +64,7 @@ TEST(Math, AlignPtr) {
 
         ASSERT_GE(aligned - unaligned, 0);
         ASSERT_LT(static_cast<size_t>(aligned - unaligned), kTestAlignment);
-        ASSERT_EQ(reinterpret_cast<intptr_t>(aligned) & (kTestAlignment -1), 0);
+        ASSERT_EQ(reinterpret_cast<uintptr_t>(aligned) & (kTestAlignment -1), 0);
     }
 }