proc_mode = PROC_EARLY; at the beginning of the function resolves the problem.
Code
proc_mode = PROC_EARLY
bmap_blit(bmapBuffer_800x600, bmapPhoto_40x40, vector(10,10,0), vector(0,0,0));  //I copy a photo into the buffer bitmap
textHalloWorld.target_map = bmapBuffer_800x600; //I copy a text into the same buffer bitmap
bmap_process(bmapShader, bmapBuffer_800x600, mtl_sepia); //use sepia shader onto bmapBuffer_800x600 and store result in bmapShader

Don't take the cake to the party.