Add MagickCore image file support to the core

MagickCore is provided here as an alternative to FFmpeg in case FFmpeg
is not easily supported (for example, debian-based linux distros).

NOTE: Cmake configuration needs to be changed in order to allow
MagickCore image support.

libobs/graphics/graphics-magick.c

+#include "graphics.h"
+
+#include <magick/MagickCore.h>
+
+void gs_init_image_deps()
+{
+	MagickCoreGenesis(NULL, MagickTrue);
+}
+
+void gs_free_image_deps()
+{
+	MagickCoreTerminus();
+}
+
+texture_t gs_create_texture_from_file(const char *file)
+{
+	texture_t     tex        = NULL;
+	ImageInfo     *info      = CloneImageInfo(NULL);
+	ExceptionInfo *exception = AcquireExceptionInfo();
+	Image         *image;
+
+	strcpy(info->filename, file);
+	image = ReadImage(info, exception);
+	if (image) {
+		size_t  cx    = image->magick_columns;
+		size_t  cy    = image->magick_rows;
+		uint8_t *data = malloc(cx * cy * 4);
+
+		ExportImagePixels(image, 0, 0, cx, cy, "BGRA", CharPixel,
+				data, exception);
+		if (exception->severity == UndefinedException)
+			tex = gs_create_texture(cx, cy, GS_BGRA, 1,
+					(const uint8**)&data, 0);
+		else
+			blog(LOG_WARNING, "magickcore warning/error getting "
+			                  "pixels from file '%s': %s", file,
+			                  exception->reason);
+
+		free(data);
+		DestroyImage(image);
+
+	} else if (exception->severity != UndefinedException) {
+		blog(LOG_WARNING, "magickcore warning/error reading file "
+		                  "'%s': %s", file, exception->reason);
+	}
+
+	DestroyImageInfo(info);
+	DestroyExceptionInfo(exception);
+
+	return tex;
+}