gles2: Honor alpha in fragment_src_external

By using the same vertex shader and adding alpha to the fragment shader
for external textures we can:

  - use alpha blending
  - have wlr_gles2_render_texture_with_matrix work with
    the GL_TEXTURE_EXTERNAL_OES. So far this failed
    since we passed in alpha which was unknown by fragment_src_external

render/gles2/renderer.c

@@ -90,7 +90,7 @@ static void init_default_shaders() {
 		goto error;
 	}
 	if (glEGLImageTargetTexture2DOES) {
-		if (!compile_program(quad_vertex_src, fragment_src_external,
+		if (!compile_program(vertex_src, fragment_src_external,
 				&shaders.external)) {
 			goto error;
 		}

render/gles2/shaders.c

@@ -80,8 +80,9 @@ const GLchar fragment_src_external[] =
 "precision mediump float;"
 "varying vec2 v_texcoord;"
 "uniform samplerExternalOES texture0;"
+"uniform float alpha;"
 ""
 "void main() {"
 "	vec4 col = texture2D(texture0, v_texcoord);"
-"	gl_FragColor = vec4(col.rgb, col.a);"
+"	gl_FragColor = vec4(col.rgb, col.a * alpha);"
 "}";