egl: Use the Mesa surfaceless platform instead of GBM

Mesa surfaceless platform appears to be a better fit for the use-case at hand:
1. Like GBM it is Mesa specific, so no change in supported setups is
   expected. If ever required, a fallback to the generic device platform
   could be added on top.
2. It leaves the complexity of selecting a renderer device to the
   driver, reducing code and dependencies.
3. It allows to use llvmpipe / software drivers without dri device,
   which can be useful on CI or debugging (with LIBGL_ALWAYS_SOFTWARE=1).

Signed-off-by: Robert Mader <robert.mader@collabora.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org> # sm8250/rb5, x1e/Dell Insprion14p
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Tested-by: Milan Zamazal <mzamazal@redhat.com> # TI AM69
Tested-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> # ThinkPad X1 Yoga Gen 7 + ov2740
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/software_isp/debayer_egl.cpp

@@ -141,8 +141,8 @@ int DebayerEGL::initBayerShaders(PixelFormat inputFormat, PixelFormat outputForm
 	egl_.pushEnv(shaderEnv, "#extension GL_OES_EGL_image_external: enable");
 
 	/*
-	 * Tell shaders how to re-order output taking account of how the
-	 * pixels are actually stored by GBM
+	 * Tell shaders how to re-order output taking account of how the pixels
+	 * are actually stored by EGL.
 	 */
 	switch (outputFormat) {
 	case formats::ARGB8888:
@@ -586,10 +586,7 @@ int DebayerEGL::start()
 {
 	GLint maxTextureImageUnits;
 
-	if (gbmSurface_.createDevice())
-		return -ENODEV;
-
-	if (egl_.initEGLContext(&gbmSurface_))
+	if (egl_.initEGLContext())
 		return -ENODEV;
 
 	glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS, &maxTextureImageUnits);

src/libcamera/software_isp/debayer_egl.h

@@ -113,7 +113,6 @@ private:
 	Rectangle window_;
 	std::unique_ptr<SwStatsCpu> stats_;
 	eGL egl_;
-	GBM gbmSurface_;
 	uint32_t width_;
 	uint32_t height_;
 	GLint glFormat_;