Avoid unnecessarily re-allocating texture buffer when the size hasn't changed.

v4:
Incorporated krh and anderco's comments.  Now adding newly allocated
buffer's dimensions to texture_damage

v3:
* Removed unnecessary parentheses
* Added check for switching from EGL image to SHM buffer
* Moved shader assignment out of IF condition

v2:
Fixed the wrong comparison

v1:
Depending on specific DRI driver implementation, glTexImage2D() with data
set to NULL may or may not re-allocate the texture buffer each time it is
called.  Unintended consequences happen if later glTexSubImage2D() is called
to only update a sub-region of the texture buffer.

I've explored moving glTexImage2D() from gl_renderer_attach() and simply
mark the texture dirty, but the current implemention seems cleaner because
I won't have to worry about calling ensure_textures() and re-assigning
gs->shader unnecessarily.

src/gl-renderer.c

@@ -1204,15 +1204,29 @@ gl_renderer_attach(struct weston_surface *es, struct wl_buffer *buffer)
 	}
 
 	if (wl_buffer_is_shm(buffer)) {
-		gs->pitch = wl_shm_buffer_get_stride(buffer) / 4;
-		gs->height = wl_shm_buffer_get_height(buffer);
-		gs->target = GL_TEXTURE_2D;
+		/* Only allocate a texture if it doesn't match existing one.
+		 * If gs->num_images is not 0, then a switch from DRM allocated
+		 * buffer to a SHM buffer is happening, and we need to allocate
+		 * a new texture buffer. */
+		if (wl_shm_buffer_get_stride(buffer) / 4 != gs->pitch ||
+		    wl_shm_buffer_get_height(buffer) != gs->height ||
+		    gs->num_images > 0) {
+			gs->pitch = wl_shm_buffer_get_stride(buffer) / 4;
+			gs->height = wl_shm_buffer_get_height(buffer);
+			gs->target = GL_TEXTURE_2D;
+
+			ensure_textures(gs, 1);
+			glBindTexture(GL_TEXTURE_2D, gs->textures[0]);
+			glTexImage2D(GL_TEXTURE_2D, 0, GL_BGRA_EXT,
+				     gs->pitch, buffer->height, 0,
+				     GL_BGRA_EXT, GL_UNSIGNED_BYTE, NULL);
+			pixman_region32_union_rect(&gs->texture_damage,
+						   &gs->texture_damage,
+						   0, 0,
+						   gs->pitch / es->buffer_scale,
+						   gs->height / es->buffer_scale);
+		}
 
-		ensure_textures(gs, 1);
-		glBindTexture(GL_TEXTURE_2D, gs->textures[0]);
-		glTexImage2D(GL_TEXTURE_2D, 0, GL_BGRA_EXT,
-			     gs->pitch, buffer->height, 0,
-			     GL_BGRA_EXT, GL_UNSIGNED_BYTE, NULL);
 		if (wl_shm_buffer_get_format(buffer) == WL_SHM_FORMAT_XRGB8888)
 			gs->shader = &gr->texture_shader_rgbx;
 		else