compositor: Implement surface global alpha

Just a small tweak to the shader and we can control the overall surface alpha.

compositor/compositor.c

@@ -225,6 +225,7 @@ wlsc_surface_create(struct wlsc_compositor *compositor,
 	surface->y = y;
 	surface->width = width;
 	surface->height = height;
+	surface->alpha = 255;
 
 	surface->fullscreen_output = NULL;
 	surface->buffer = NULL;
@@ -548,6 +549,12 @@ wlsc_surface_draw(struct wlsc_surface *es,
 		break;
 	}
 
+	if (es->alpha != ec->current_alpha) {
+		glUniform1f(ec->texture_shader.alpha_uniform,
+			    es->alpha / 255.0);
+		ec->current_alpha = es->alpha;
+	}
+
 	if (es->transform == NULL) {
 		filter = GL_NEAREST;
 		n = texture_region(es, &repaint);
@@ -1626,9 +1633,11 @@ static const char texture_fragment_shader[] =
 	"precision mediump float;\n"
 	"varying vec2 v_texcoord;\n"
 	"uniform sampler2D tex;\n"
+	"uniform float alpha;\n"
 	"void main()\n"
 	"{\n"
 	"   gl_FragColor = texture2D(tex, v_texcoord)\n;"
+	"   gl_FragColor = alpha * gl_FragColor;\n"
 	"}\n";
 
 static const char solid_fragment_shader[] =
@@ -1687,6 +1696,7 @@ wlsc_shader_init(struct wlsc_shader *shader,
 
 	shader->proj_uniform = glGetUniformLocation(shader->program, "proj");
 	shader->tex_uniform = glGetUniformLocation(shader->program, "tex");
+	shader->alpha_uniform = glGetUniformLocation(shader->program, "alpha");
 
 	return 0;
 }

compositor/compositor.h

@@ -128,6 +128,7 @@ struct wlsc_shader {
 	GLuint vertex_shader, fragment_shader;
 	GLuint proj_uniform;
 	GLuint tex_uniform;
+	GLuint alpha_uniform;
 	GLuint color_uniform;
 };
 
@@ -173,7 +174,8 @@ struct wlsc_compositor {
 	EGLContext context;
 	EGLConfig config;
 	GLuint fbo;
-	GLuint proj_uniform, tex_uniform;
+	GLuint proj_uniform, tex_uniform, alpha_uniform;
+	uint32_t current_alpha;
 	struct wlsc_shader texture_shader;
 	struct wlsc_shader solid_shader;
 	struct wl_display *wl_display;
@@ -241,6 +243,7 @@ struct wlsc_surface {
 	struct wl_list link;
 	struct wl_list buffer_link;
 	struct wlsc_transform *transform;
+	uint32_t alpha;
 	uint32_t visual;
 
 	/*