implemented some of the camera functions

src/camera.c

@@ -0,0 +1,20 @@
+#include "camera.h"
+#include "render.h"
+
+Camera g_camera;
+
+void camera_init() {
+    g_camera.centre = ZeroVector;
+    g_camera.width = 1;
+}
+
+SDL_FRect render_calculate_unit_rect() {
+    // If the camera's width is 1, this rectangle should fill the width of the screen.
+    // If the camera's width is 2, this rectangle should fill half the width of the screen.
+    return (SDL_FRect) {
+        .x = -g_camera.centre.x * g_render_resolution.x,
+        .y = -g_camera.centre.y * g_render_resolution.x,
+        .w = g_render_resolution.x / g_camera.width,
+        .h = g_render_resolution.x / g_camera.width
+    };
+}