[le_screenshot] implement automatic name offset calculation

if the screenshot module detects that there are already images that
match the current format string in the target path, then we find out the
highest saved frame number and use the next number as our starting
offset for any new sequence of saved images.

modules/le_screenshot/le_screenshot.cpp

@@ -7,6 +7,7 @@
 #include "le_swapchain_img.h"
 #include "le_png.h"
 #include "le_log.h"
+#include <filesystem>
 
 static constexpr auto LOGGER_LABEL = "le_screenshot";
 
@@ -215,6 +216,37 @@ static bool le_screenshot_record( le_screenshot_o* self, le_rendergraph_o* rg, l
 			self->swapchain_settings.height_hint = fallback_height;
 		};
 
+		{
+
+			// We scan the target directory for existing screenshots - if screenshots exist,
+			// then we will try to find the one with the highest number matching our labelling scheme
+			// and start our new screenshots at this offset.
+
+			auto        target_path    = std::filesystem::path( self->swapchain_settings.image_filename_template ).remove_filename();
+			std::string ext            = std::filesystem::path( self->swapchain_settings.image_filename_template ).extension();
+			uint32_t    largest_number = 0;
+
+			// iterate over all files in the target path -- this can get slow if there are lots of files in there.
+			for ( auto const& f : std::filesystem::directory_iterator{ target_path } ) {
+				if ( std::filesystem::is_regular_file( f ) && f.path().extension() == ext ) {
+
+					logger.debug( "Found existing screenshot: %s", f.path().c_str() );
+
+					uint32_t frame_number = 0;
+					if ( 1 == sscanf( f.path().c_str(), self->swapchain_settings.image_filename_template, &frame_number ) ) {
+
+						if ( frame_number >= largest_number ) {
+							largest_number = frame_number + 1;
+						}
+					}
+				}
+			}
+
+			self->swapchain_settings.frame_number_offset = largest_number;
+
+			logger.info( "Starting Screenshot numbering at: %08d", self->swapchain_settings.frame_number_offset );
+		}
+
 		self->swapchain = le_renderer_api_i->le_renderer_i.add_swapchain( self->renderer, self->swapchain_settings );
 	}