Specify camera FOV instead of viewport height

src/camera.c

@@ -9,6 +9,10 @@
 #include <stdio.h>
 #include <threads.h>
 
+#ifndef M_PI
+#define M_PI 3.14159265258979323846264
+#endif
+
 #define MAX_ITER 10
 #define MIN_T 1e-6
 #define SAMPLE_STDDEV 0.333
@@ -113,12 +117,16 @@ static int render_thread(void *arg)
 }
 
 camera_t camera_init(
-    vec3_t pos, double focal_len, double viewport_height, uint32_t img_width,
+    vec3_t pos, double focal_len, double fov, uint32_t img_width,
     uint32_t img_height, unsigned samples_per_pixel)
 {
 	const double aspect = (double)img_width / (double)img_height;
+
+	const double fov_rad = M_PI * fov / 180.0;
+	const double viewport_height = focal_len * tan(fov_rad / 2);
 	const double viewport_width = viewport_height * aspect;
 	const vec3_t viewport_disp = { 0, 0, focal_len };
+
 	const vec3_t u = { viewport_width, 0, 0 };
 	const vec3_t v = { 0, -viewport_height, 0 };
 	const vec3_t topleft = vec3_sub(