fosphor/gl: Use glTexSubImage2D for texture update

Much faster than glTexImage2D since it only updates the data
and doesn't delete/create a new storage space for the data.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

lib/fosphor/gl.c

@@ -90,10 +90,9 @@ gl_tex2d_write(GLuint tex_id, float *src, int width, int height)
 {
 	glBindTexture(GL_TEXTURE_2D, tex_id);
 
-	glTexImage2D(
-		GL_TEXTURE_2D,
-		0, GL_R32F,
-		width, height, 0,
+	glTexSubImage2D(
+		GL_TEXTURE_2D, 0,
+		0, 0, width, height,
 		GL_RED, GL_FLOAT,
 		src
 	);