Make scalar plain C

This removes the need for a C++ compiler to be installed.
It is much more likely for a C compiler to be installed on someone's Linux machine in comparison.

build.rs

@@ -45,7 +45,7 @@ fn main() {
     } else if !env.contains("windows") && !env.contains("wasm32") {
         // build scalar on non-windows and non-mac
         cc::Build::new()
-            .file("src/native/posix/scalar.cpp")
+            .file("src/native/posix/scalar.c")
             .opt_level(3) // always build with opts for scaler so it's fast in debug also
             .compile("libscalar.a")
     }

src/native/posix/scalar.cpp → src/native/posix/scalar.c

@@ -1,7 +1,6 @@
 #include <stdint.h>
-#include <stdio.h>
 
-extern "C" void Image_resize_linear_c(uint32_t* target, const uint32_t* source, int w, int h, int s, int w2, int h2) {
+void Image_resize_linear_c(uint32_t* target, const uint32_t* source, int w, int h, int s, int w2, int h2) {
     if (w2 <= 0) { w2 = 1; }
     if (h2 <= 0) { h2 = 1; }
 
@@ -51,7 +50,7 @@ static void resize_linear_c_stride(uint32_t* target, const uint32_t* source, int
     }
 }
 
-extern "C" void Image_resize_linear_aspect_fill_c(
+void Image_resize_linear_aspect_fill_c(
     uint32_t* target,
     const uint32_t* source,
     int w, int h, int s,
@@ -62,8 +61,8 @@ extern "C" void Image_resize_linear_aspect_fill_c(
         target[i] = bg_clear;
     }
 
-    float buffer_aspect = float(w) / float(h);
-    float win_aspect = float(window_width) / float(window_height);
+    float buffer_aspect = (float)w / (float)h;
+    float win_aspect = (float)window_width / (float)window_height;
 
     if (buffer_aspect > win_aspect) {
         int new_height = (int)(window_width / buffer_aspect);
@@ -81,7 +80,7 @@ extern "C" void Image_resize_linear_aspect_fill_c(
     }
 }
 
-extern "C" void Image_center(
+void Image_center(
     uint32_t* target,
     const uint32_t* source,
     int w, int h, int s,
@@ -155,7 +154,7 @@ extern "C" void Image_center(
     }
 }
 
-extern "C" void Image_upper_left(
+void Image_upper_left(
     uint32_t* target,
     const uint32_t* source,
     int w, int h, int s,