Headless: force opaque PNG screenshots by default, add --screenshot-keep-alpha

Games often use the framebuffer alpha channel for non-visual purposes,
so saved PNGs could look fully transparent in image viewers. Force alpha
to 255 when writing PNG screenshots unless --screenshot-keep-alpha is
passed. The MSE comparison ignores alpha either way.

Core/CmdLine.cpp

@@ -183,6 +183,7 @@ static const CommandLineParam g_autoParams[] = {
 	{POFF(screenshotFilename), CmdParamType::String, "screenshot", '\0', "Compare rendered output against a reference screenshot FILE", CmdLineMode::Headless},
 	{POFF(screenshotFilenameSave), CmdParamType::String, "screenshot-save", '\0', "Save rendered screenshot to specified path (PNG if the path ends in .png, BMP otherwise)", CmdLineMode::Headless},
 	{POFF(screenshotFilenameDiff), CmdParamType::String, "screenshot-diff", '\0', "Save a visual comparison image to FILE when comparing screenshots", CmdLineMode::Headless},
+	{POFF(screenshotSaveKeepAlpha), CmdParamType::Bool, "screenshot-keep-alpha", '\0', "Preserve the alpha channel when saving PNG screenshots (default: alpha is forced to 255)", CmdLineMode::Headless},
 	{POFF(timeout), CmdParamType::Double, "timeout", '\0', "Set the timeout value", CmdLineMode::Headless},
 	{POFF(maxScreenshotError), CmdParamType::Double, "max-mse", '\0', "Maximum allowed MSE error for screenshot comparison", CmdLineMode::Headless},
 	{POFF(mountIso), CmdParamType::String, "mount", 'm', "Mount ISO/CSO on umd1:", CmdLineMode::Headless},