Readback stencil buffer for debugger on GLES

[unknownbrackets]

This allows reading the stencil buffer via the API (so now depth and stencil both work) under GLES 3.1+. Was very straight-forward given the depth readback.

Just for example, a simple nodejs script to grab stencil (see ppsspp-api-samples):

const PPSSPP = require('./sdk');
const ppsspp = new PPSSPP();
const fs = require('fs');

main();

async function main() {
	try {
		await ppsspp.autoConnect();
		await ppsspp.send({ event: 'version', name: 'stencil-screenshot', version: '1.0.0' });

		// Need to pause emulation to take a screenshot.
		await ppsspp.send({ event: 'cpu.stepping' });
		const result = await ppsspp.send({ event: 'gpu.buffer.renderStencil', type: 'uri', alpha: false });
		await ppsspp.send({ event: 'cpu.resume' });

		const dataPrefix = 'data:image/png;base64,';
		if (!result.uri || result.uri.substr(0, dataPrefix.length) !== dataPrefix) {
			throw 'Unexpected data URI returned';
		}

		const buffer = Buffer.from(result.uri.substr('data:image/png;base64,'.length), 'base64');
		fs.writeFileSync('screenshot.png', buffer);
	} catch (err) {
		console.error('Something went wrong:', err);
	} finally {
		try {
			ppsspp.disconnect();
		} catch (err) {
			console.error('Could not disconnect:', err);
		}
	}
}

-[Unknown]

[hrydgard]

I see you're sending "alpha: false" in the example code, does that affect things when reading stencil?

[unknownbrackets]

I see you're sending "alpha: false" in the example code, does that affect things when reading stencil?

Yes, when using the debugger APIs, alpha: true means the red channel is always 100% and the alpha channel represents the amount of stencil (or depth.) And alpha: false means the red channel is the amount of stencil/depth.

-[Unknown]