image-source: Increase slideshow limit to 400MB

Increases the image slideshow size limit -- this is a temporary solution
until a newer revision of the image slideshow can be written.

plugins/image-source/obs-slideshow.c

@@ -64,7 +64,7 @@
 extern uint64_t image_source_get_memory_usage(void *data);
 
 #define BYTES_TO_MBYTES (1024 * 1024)
-#define MAX_MEM_USAGE (250 * BYTES_TO_MBYTES)
+#define MAX_MEM_USAGE (400 * BYTES_TO_MBYTES)
 
 struct image_file_data {
 	char *path;