Remove macros Vulkan shader definitions

[louwers]

I am getting compile errors related to the shaders in include/mbgl/shaders/vulkan

  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: error: unterminated raw string
     10 |     R"(
        |     ^
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:15:2: warning: missing terminating " character
     15 | )"
        |  ^
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:15:2: error: missing terminating " character
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: error: stray 'R' in program
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: note: in definition of macro 'BACKGROUND_SHADER_COMMON'
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: error: stray 'R' in program
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: note: in definition of macro 'BACKGROUND_SHADER_COMMON'
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: error: stray 'R' in program
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: note: in definition of macro 'BACKGROUND_SHADER_COMMON'
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: error: stray 'R' in program
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:10:5: note: in definition of macro 'BACKGROUND_SHADER_COMMON'
     10 |     R"(
        |     ^~~
  /home/bart/maplibre-native/include/mbgl/shaders/vulkan/background.hpp:15:1: error: expected unqualified-id before ')' token
     15 | )"

We should refactor this code so that we do not have to use #define.