backend: gl: fix shadow from mask

The intermediate texture used for shadow from mask calculation did not
properly set the min/mag filter to linear, which is required by the blur
methods. Because they use texture interpolation to accelerate
the convolution calculation.

Fixes #916

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/gl_common.c

@@ -1213,6 +1213,8 @@ void *gl_shadow_from_mask(backend_t *base, void *mask,
 	auto source_texture = gl_new_texture(GL_TEXTURE_2D);
 	glActiveTexture(GL_TEXTURE0);
 	glBindTexture(GL_TEXTURE_2D, source_texture);
+	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 	glTexImage2D(GL_TEXTURE_2D, 0, GL_RED, new_inner->width, new_inner->height, 0,
 	             GL_RED, GL_UNSIGNED_BYTE, NULL);
 	glBindTexture(GL_TEXTURE_2D, 0);